I am not advocating usingexpired yeastsbutI have successfully used yeast over a year expired with good results. It was kept refrigerated the entire time I have had it as are all of my yeasts. As Wade states, if you have to use an expired yeast, make a starter to asses the viability of the yeast. Even re-hydrating will help give it a jump start if for some reason you can't make a starter. Even if it isn't expired but may be getting close, rehydrate or make a starter for a little extra assurance.