On average, mothers earn lower wages than childless women. This well-established finding is referred to as the “motherhood penalty.” In this review, we summarize the main theoretical explanations for the motherhood penalty, and briefly discuss which theories have received empirical support. We evaluate research that explores variation in the motherhood penalty by important demographic and job-related characteristics. Additionally, we highlight recent methodological advances used to estimate the penalty. The review concludes with suggestions for future research in this area.