Many problems have been reported concerning null hypothesis significance testing (NHST), although it is used in most psychological research. The authors believe that this situation will not change, at least not in the near future. Before shifting to a Bayesian approach, there are several things that must be done in psychological research that uses NHST. Among them, sample size planning (SSP) is especially important. In practice, whether significant results exist primarily depends on sample size, and many studies have indicated that SSP is vital. However, the use of SSP is rare in psychological research. Most psychological research basically depends on the significance of NHST; therefore, psychologists should conduct SSP before collecting data. We describe the present situation of SSP in psychological research, discuss related topics, argue for the importance of SSP, and present future directions for SSP in psychological research using NHST.