As a foundation for muscle gains, quality protein supplementation is without equal. And the best absorbed form - often used specifically post-workout, and with meals to round out one's protein balance - is whey protein. Whey protein has a high biological value and is extremely convenient to take.
While protein obtained from food sources can provide more sustained protein levels (especially prior to bed to offset the fasting period that usually occurs during sleep), whey is rapidly shuttled into the muscles where it can exert its powerful anabolic effects.