diff --git a/Sources/TSCBasic/Process.swift b/Sources/TSCBasic/Process.swift
index 5a396823..d8619e98 100644
--- a/Sources/TSCBasic/Process.swift
+++ b/Sources/TSCBasic/Process.swift
@@ -23,6 +23,13 @@ import Dispatch
 
 import _Concurrency
 
+#if os(Windows)
+public typealias ProcessEnvironmentBlock = Dictionary<CaseInsensitiveString, String>
+#else
+public typealias ProcessEnvironmentBlock = Dictionary<String, String>
+#endif
+
+
 /// Process result data which is available after process termination.
 public struct ProcessResult: CustomStringConvertible, Sendable {
 
@@ -53,7 +60,7 @@ public struct ProcessResult: CustomStringConvertible, Sendable {
     public let arguments: [String]
 
     /// The environment with which the process was launched.
-    public let environment: [String: String]
+    public let environment: ProcessEnvironmentBlock
 
     /// The exit status of the process.
     public let exitStatus: ExitStatus
@@ -71,7 +78,7 @@ public struct ProcessResult: CustomStringConvertible, Sendable {
     /// See `waitpid(2)` for information on the exit status code.
     public init(
         arguments: [String],
-        environment: [String: String],
+        environment: ProcessEnvironmentBlock,
         exitStatusCode: Int32,
         normal: Bool,
         output: Result<[UInt8], Swift.Error>,
@@ -99,7 +106,7 @@ public struct ProcessResult: CustomStringConvertible, Sendable {
     /// Create an instance using an exit status and output result.
     public init(
         arguments: [String],
-        environment: [String: String],
+        environment: ProcessEnvironmentBlock,
         exitStatus: ExitStatus,
         output: Result<[UInt8], Swift.Error>,
         stderrOutput: Result<[UInt8], Swift.Error>
@@ -285,7 +292,7 @@ public final class Process {
     public let arguments: [String]
 
     /// The environment with which the process was executed.
-    public let environment: [String: String]
+    public let environment: ProcessEnvironmentBlock
 
     /// The path to the directory under which to run the process.
     public let workingDirectory: AbsolutePath?
@@ -359,7 +366,7 @@ public final class Process {
     @available(macOS 10.15, *)
     public init(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         workingDirectory: AbsolutePath,
         outputRedirection: OutputRedirection = .collect,
         startNewProcessGroup: Bool = true,
@@ -379,7 +386,7 @@ public final class Process {
     @available(macOS 10.15, *)
     public convenience init(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         workingDirectory: AbsolutePath,
         outputRedirection: OutputRedirection = .collect,
         verbose: Bool,
@@ -411,7 +418,7 @@ public final class Process {
     ///   - loggingHandler: Handler for logging messages
     public init(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         outputRedirection: OutputRedirection = .collect,
         startNewProcessGroup: Bool = true,
         loggingHandler: LoggingHandler? = .none
@@ -428,7 +435,7 @@ public final class Process {
     @available(*, deprecated, message: "use version without verbosity flag")
     public convenience init(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         outputRedirection: OutputRedirection = .collect,
         verbose: Bool = Process.verbose,
         startNewProcessGroup: Bool = true
@@ -444,7 +451,7 @@ public final class Process {
 
     public convenience init(
         args: String...,
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         outputRedirection: OutputRedirection = .collect,
         loggingHandler: LoggingHandler? = .none
     ) {
@@ -536,7 +543,13 @@ public final class Process {
             process.currentDirectoryURL = workingDirectory.asURL
         }
         process.executableURL = executablePath.asURL
+#if os(Windows)
+        process.environment = .init(uniqueKeysWithValues: environment.map {
+            ($0.value, $1)
+        })
+#else
         process.environment = environment
+#endif
 
         let stdinPipe = Pipe()
         process.standardInput = stdinPipe
@@ -989,7 +1002,7 @@ extension Process {
     @available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
     static public func popen(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) async throws -> ProcessResult {
         let process = Process(
@@ -1012,7 +1025,7 @@ extension Process {
     @available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
     static public func popen(
         args: String...,
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) async throws -> ProcessResult {
         try await popen(arguments: args, environment: environment, loggingHandler: loggingHandler)
@@ -1030,7 +1043,7 @@ extension Process {
     @discardableResult
     static public func checkNonZeroExit(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) async throws -> String {
         let result = try await popen(arguments: arguments, environment: environment, loggingHandler: loggingHandler)
@@ -1053,7 +1066,7 @@ extension Process {
     @discardableResult
     static public func checkNonZeroExit(
         args: String...,
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) async throws -> String {
         try await checkNonZeroExit(arguments: args, environment: environment, loggingHandler: loggingHandler)
@@ -1075,7 +1088,7 @@ extension Process {
 //    #endif
     static public func popen(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none,
         queue: DispatchQueue? = nil,
         completion: @escaping (Result<ProcessResult, Swift.Error>) -> Void
@@ -1113,7 +1126,7 @@ extension Process {
     @discardableResult
     static public func popen(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) throws -> ProcessResult {
         let process = Process(
@@ -1140,7 +1153,7 @@ extension Process {
     @discardableResult
     static public func popen(
         args: String...,
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) throws -> ProcessResult {
         return try Process.popen(arguments: args, environment: environment, loggingHandler: loggingHandler)
@@ -1160,7 +1173,7 @@ extension Process {
     @discardableResult
     static public func checkNonZeroExit(
         arguments: [String],
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) throws -> String {
         let process = Process(
@@ -1192,7 +1205,7 @@ extension Process {
     @discardableResult
     static public func checkNonZeroExit(
         args: String...,
-        environment: [String: String] = ProcessEnv.vars,
+        environment: ProcessEnvironmentBlock = ProcessEnv.vars,
         loggingHandler: LoggingHandler? = .none
     ) throws -> String {
         return try checkNonZeroExit(arguments: args, environment: environment, loggingHandler: loggingHandler)
diff --git a/Sources/TSCBasic/ProcessEnv.swift b/Sources/TSCBasic/ProcessEnv.swift
index bf7e3c62..90639916 100644
--- a/Sources/TSCBasic/ProcessEnv.swift
+++ b/Sources/TSCBasic/ProcessEnv.swift
@@ -9,25 +9,74 @@
 */
 
 import Foundation
+#if os(Windows)
+import WinSDK
+#else
 import TSCLibc
+#endif
+
+public struct CaseInsensitiveString {
+    internal var value: String
+
+    public init(_ value: String) {
+        self.value = value
+    }
+}
+
+extension CaseInsensitiveString: ExpressibleByStringLiteral {
+    public init(stringLiteral value: String) {
+        self.value = value
+    }
+}
+
+extension CaseInsensitiveString: Equatable {
+    public static func == (_ lhs: Self, _ rhs: Self) -> Bool {
+        return lhs.value.lowercased() == rhs.value.lowercased()
+    }
+}
+
+extension CaseInsensitiveString: Hashable {
+    public func hash(into hasher: inout Hasher) {
+        self.value.lowercased().hash(into: &hasher)
+    }
+}
 
 /// Provides functionality related a process's enviorment.
 public enum ProcessEnv {
 
     /// Returns a dictionary containing the current environment.
+#if os(Windows)
+    public static var vars: [CaseInsensitiveString: String] { _vars }
+    private static var _vars: Dictionary<CaseInsensitiveString, String> = {
+        .init(uniqueKeysWithValues: ProcessInfo.processInfo.environment.map {
+            (CaseInsensitiveString($0), $1)
+        })
+    }()
+#else
     public static var vars: [String: String] { _vars }
     private static var _vars = ProcessInfo.processInfo.environment
+#endif
 
     /// Invalidate the cached env.
     public static func invalidateEnv() {
+#if os(Windows)
+        _vars = .init(uniqueKeysWithValues: ProcessInfo.processInfo.environment.map {
+            (CaseInsensitiveString($0), $1)
+        })
+#else
         _vars = ProcessInfo.processInfo.environment
+#endif
     }
 
     /// Set the given key and value in the process's environment.
     public static func setVar(_ key: String, value: String) throws {
       #if os(Windows)
-        guard TSCLibc._putenv("\(key)=\(value)") == 0 else {
-            throw SystemError.setenv(Int32(GetLastError()), key)
+        try key.withCString(encodedAs: UTF16.self) { pwszKey in
+            try value.withCString(encodedAs: UTF16.self) { pwszValue in
+                guard SetEnvironmentVariableW(pwszKey, pwszValue) else {
+                    throw SystemError.setenv(Int32(GetLastError()), key)
+                }
+            }
         }
       #else
         guard TSCLibc.setenv(key, value, 1) == 0 else {
@@ -40,7 +89,9 @@ public enum ProcessEnv {
     /// Unset the give key in the process's environment.
     public static func unsetVar(_ key: String) throws {
       #if os(Windows)
-        guard TSCLibc._putenv("\(key)=") == 0 else {
+        guard (key.withCString(encodedAs: UTF16.self) {
+            SetEnvironmentVariableW($0, nil)
+        }) else {
             throw SystemError.unsetenv(Int32(GetLastError()), key)
         }
       #else
@@ -54,11 +105,10 @@ public enum ProcessEnv {
     /// `PATH` variable in the process's environment (`Path` under Windows).
     public static var path: String? {
 #if os(Windows)
-        let pathArg = "Path"
+        return vars["Path"]
 #else
-        let pathArg = "PATH"
+        return vas["PATH"]
 #endif
-        return vars[pathArg]
     }
 
     /// The current working directory of the process.
@@ -70,9 +120,7 @@ public enum ProcessEnv {
     public static func chdir(_ path: AbsolutePath) throws {
         let path = path.pathString
       #if os(Windows)
-        guard path.withCString(encodedAs: UTF16.self, {
-            SetCurrentDirectoryW($0)
-        }) else {
+        guard path.withCString(encodedAs: UTF16.self, SetCurrentDirectoryW) else {
             throw SystemError.chdir(Int32(GetLastError()), path)
         }
       #else