This study examines the potential role of further increases in contraceptive prevalence and effectiveness in reducing abortion rates. The model used in this analysis links the abortion rate to its direct determinants, including couples' reproductive preferences, the prevalence and effectiveness of contraceptive practice to implement these preferences, and the probability of undergoing an abortion to avoid an unintended birth when a contraceptive fails or is not used. An assessment of the tradeoff between contraception and abortion yields estimates of the decline in the total abortion rate that would result from an illustrative increase of 10 percentage points in prevalence. This effect varies among societies, primarily because the tendency to obtain an abortion after an unintended pregnancy varies. For example, in a population with an abortion probability of 0.5, a 10 percentage-point increase in prevalence would avert approximately 0.45 abortions per woman, assuming contraception is 95 percent effective. If all unintended pregnancies were aborted, this effect would be three times larger. Eliminating all unintended pregnancies and subsequent abortions would require a rise in contraceptive prevalence to the level at which all fecund women who do not wish to become pregnant practice contraception that is 100 percent effective. A procedure is provided for estimating this "perfect" level of contraceptive prevalence.