Parental involvement research has greatly expanded over the past decade, but findings are mixed, reflecting in part the conceptual and methodological limitations of many studies. On the basis of longitudinal questionnaire data from 1,685 sixth-grade students, the authors studied parental help with homework because it is the most common and most controversial type of parental involvement. Distinguishing between the quantity and quality of parental homework involvement, the research shows that completely different conclusions about the effectiveness of parental homework involvement will be reached if its quantity is assessed instead of its quality: How often parents helped with homework was negatively associated with the development of achievement, whereas homework help that was perceived as supportive had positive predictive effects, and homework help perceived as intrusive had negative effects. Moreover, the results show that effect sizes would be overestimated if students' prior achievement and family background were not controlled.