While most practitioners and researchers support the recent policy direction for increased parent involvement in their children's education, little consensus exists about what constitutes effective parent involvement. A major source of this confusion is the lack of scientific rigor in the research upon which practice and policy is based. This literature review examines over 200 articles on parent involvement to highlight what is known and to point to significant gaps in research, programs, and practice. Papers reviewed include 23 opinion papers and program descriptions, 13 theoretical papers, 30 literature reviews, and 145 empirical studies. The review points to significant problems with the literature. Specifically, non-experimental designs, not testing for the specific effects of parent involvement, inconsistent operational definitions of parent involvement, and reliance on non-objective measures are four methodological limitations which have compromised the rigor of much parent involvement research. It concludes with seven recommendations for future research, including use of experimental procedures (especially random assignment), making explicit which aspect of parent involvement is being measured, and use of objective measurement such as direct observation and standardized data collection tools. Contains approximately 211 references. (EV)