Hi, I'm using GHC 6.6 (debian/etch) - and having some fun with threadDelay. When compiled without the -threaded compiler argument it "behaves" as documented - waits at least the interval - for example: Tgt/Actual = 0.000000/0.036174s, diff = 0.036174s Tgt/Actual = 0.000001/0.049385s, diff = 0.049384s Tgt/Actual = 0.000005/0.049492s, diff = 0.049487s Tgt/Actual = 0.000025/0.049596s, diff = 0.049571s Tgt/Actual = 0.000125/0.049655s, diff = 0.04953s Tgt/Actual = 0.000625/0.04969s, diff = 0.049065s Tgt/Actual = 0.003125/0.049684s, diff = 0.046559s Tgt/Actual = 0.015625/0.04962s, diff = 0.033995s Tgt/Actual = 0.078125/0.099668s, diff = 0.021543s Tgt/Actual = 0.390625/0.399637s, diff = 0.009012s Tgt/Actual = 1.953125/1.999515s, diff = 0.04639s Tgt/Actual = 9.765625/9.799505s, diff = 0.03388s however when -threaded is used you get some interesting effects, including returning too early: Tgt/Actual = 0.000000/0.000093s, diff = 0.000093s Tgt/Actual = 0.000001/0.000031s, diff = 0.00003s Tgt/Actual = 0.000005/0.000029s, diff = 0.000024s Tgt/Actual = 0.000025/0.000028s, diff = 0.000003s Tgt/Actual = 0.000125/0.000034s, diff = -0.000091s Tgt/Actual = 0.000625/0.000035s, diff = -0.00059s Tgt/Actual = 0.003125/0.000029s, diff = -0.003096s Tgt/Actual = 0.015625/0.000028s, diff = -0.015597s Tgt/Actual = 0.078125/0.058525s, diff = -0.0196s Tgt/Actual = 0.390625/0.389669s, diff = -0.000956s Tgt/Actual = 1.953125/1.939513s, diff = -0.013612s Tgt/Actual = 9.765625/9.749573s, diff = -0.016052s The program I used to generate this is :- import Control.Concurrent import Data.Time import Text.Printf main = mapM_ delay (0 : take 11 (iterate (*5) 1)) delay n = do tS <- getCurrentTime threadDelay n tE <- getCurrentTime let n' = toRational n / 10^6 n'' = fromRational (n') :: Double obs = diffUTCTime tE tS printf "Tgt/Actual = %0.6f/%s, diff = %s\n" n'' (show obs) (show $ obs - fromRational n') return () Any suggestions? Neil