Motherhood is associated with lower hourly pay, but the causes of this are not well understood. Mothers may earn less than other women because having children causes them to (1) lose job experience, (2) be less productive at work, (3) trade off higher wages for mother-friendly jobs, or (4) be discriminated against by employers. Or the relationship may be spurious rather than causal—women with lower earning potential may have children at relatively higher rates. The authors use data from the 1982–1993 National Longitudinal Survey of Youth with fixed-effects models to examine the wage penalty for motherhood. Results show a wage penalty of 7 percent per child. Penalties are larger for married women than for unmarried women. Women with (more) children have fewer years of job experience, and after controlling for experience a penalty of 5 percent per child remains. “Mother-friendly” characteristics of the jobs held by mothers explain little of the penalty beyond the tendency of more mothers than non-mothers to work part-time. The portion of the motherhood penalty unexplained probably results from the effect of motherhood on productivity and/or from discrimination by employers against mothers. While the benefits of mothering diffuse widely—to the employers, neighbors, friends, spouses, and children of the adults who received the mothering—the costs of child rearing are borne disproportionately by mothers.