Studies of simple visual and auditory reaction processes published since Teichner are reviewed, with emphasis on the preparatory phase of these processes, particularly the foreperiod (FP). The first section consists of the analysis of actual FP variables: duration, regularity, range, distribution, and preceding FPs. The second section deals with factors affecting the relation between FP and reaction time (RT): (a) the modality, intensity, probability, and duration of the warning signal and reaction stimulus (RS) and (b) the speed-accuracy trade-off. It is suggested that the subject's expectancy (momentary probability of the immediate delivery of the RS) is the most important determinant of the preparation to respond to the RS at any moment during a trial and, hence, of RT. Many factors exert their influence on preparation through their effect on expectancy. Other factors have a direct influence on preparation: short-term exhaustion and fatigue, immediate arousal evoked by intense auditory stimuli, the subject's stimulus criterion, the speed-accuracy trade-off, and fluctuation in motor preparedness owing to inaccuracies in the control of motor preparation.