The effect of stress on soybean yield is determined, in large part, by when the stress occurs. Dividing the plant’s life cycle into three phases (vegetative growth, flowering and pod set, and seed filling) provides a convenient framework for evaluating timing effects. Stress during vegetative growth affects yield only if it reduces light interception during reproductive growth. Soybean can often tolerate stress during this phase because large LAIs prevent reductions in light interception. Stress during flowering and podset probably has the largest potential affect on yield because pod and seed number are the most important yield components. The long flowering and pod set period (up to 30 or 40 days) is often thought to provide some tolerance to short periods of stress. Most pods, however, are produced in roughly half of the total period and the plant may not have the capacity to completely recover from stress during part or all of this relatively short critical period. Increases in seed size (weight per seed) may counter some of the reductions in seed number if stress is relieved during seed filling. Stress during seed filling reduces seed size and yield as a result of a shorter seed filling period (water stress) or reductions in seed growth rate (shade stress). Any evaluation of stress effects on soybean yield must consider both the level of stress and when it occurs to be successful.