The purpose of this paper is to promote statistical power analysis in the behavioral sciences by introducing the easy to use GPower software. GPower is a free general power analysis program available in two essentially equivalent versions, one designed for Macintosh OS/OS X and the other for MS-DOS/Windows platforms. Psychological research examples are presented to illustrate the various features of the GPower software. In particular, a priori, post-hoc, and compromise power analyses for t-tests, F-tests, and chi-2-tests will be demonstrated. For all examples, the underlying statistical concepts as well as the implementation in GPower will be described.