diff --git a/ue4-rebuild-lightmaps.ps1 b/ue4-rebuild-lightmaps.ps1
index 096d2d7..5488c41 100644
--- a/ue4-rebuild-lightmaps.ps1
+++ b/ue4-rebuild-lightmaps.ps1
@@ -34,6 +34,13 @@ function Write-Usage {
     Write-Output "                  rebuild based on cooked maps in packageconfig.json"
     Write-Output "  -dryrun       : Don't perform any actual actions, just report on what you would do"
     Write-Output "  -help         : Print this help"
+    Write-Output " "
+    Write-Output "Environment Variables:"
+    Write-Output "  UE4INSTALL   : Use a specific UE4 install."
+    Write-Output "               : Default is to find one based on project version, under UE4ROOT"
+    Write-Output "  UE4ROOT      : Parent folder of all binary UE4 installs (detects version). "
+    Write-Output "               : Default C:\Program Files\Epic Games"
+    Write-Output " "
 }
 
 if ($src.Length -eq 0) {