Objectives: To provide an overview of contraceptive effectiveness and its determinants as well as reasons for the gap between the actual and expected number of unplanned pregnancies for various categories of users; to provide some guidelines for more precise, consistent terminology and measurements for use in future research; and to provide specific counseling guidelines; using combined oral contraceptives (OCs) as the example. Data Sources: Fifty-three articles on contraceptive effectiveness were reviewed, with a particular focus on pregnancy rates among OC users and the quality of use contributing to those rates. Definitions and measurement of contraceptive effectiveness, its components, and its determinants were examined and discrepancies explored. Methods of Study Selection: Papers discussed were selected because they had special relevance. Only a few of the papers discussed in this article were quantitative studies of contraceptive behavior and effectiveness. Others were review articles, discussions of the appropriate measurement of various components of effectiveness. Others provided conceptual frameworks for studying method effectiveness. All published studies of OC compliance were reviewed. Tabulation, Integration, and Results: Comparing the definitions and measurement of both compliance and effectiveness revealed important inconsistencies. However, some trends did emerge, and these papers did provide a framework for the following discussion. The findings were used to develop a more coherent set of definitions, measurement approaches, and counseling strategies. Characteristics of the user, the method, and the service delivery system all play a role in contraceptive effectiveness. However, it is the quality of use, ie, the user's own behavior, that is the immediate determinant of the effectiveness of any short-term contraceptive method. The mean pregnancy rate for better-than-average users is 4%; for poorer-than-average users, it is 8%. Conclusions: The range of contraceptive effectiveness rates is closely associated with user characteristics. Oral contraceptive users need to know that their own chance of becoming pregnant will vary with their fecundability, when and how frequently they have sexual intercourse, and, especially, how consistently and correctly they take their pills. The likelihood that any one user will take OCs correctly and consistently also is associated with her life-style, socioeconomic status, age, and other factors. Further research is needed to determine how forgiving OCs are of various pill-taking errors, to determine the impact of dual method use (eg, OCs and condoms), and to assess the effects of user, method, and programmatic characteristics on correct and consistent use and, in turn, on effectiveness.