In the context of a clinical trial, adherence refers to the extent to which study participants use the assigned study products or follow study regimens as directed. It can be measured through self-reports, through direct observation, or by using a variety of more objective measures. Adherence data are important for accurately assessing the safety and effectiveness of the study products. Data on why participants did or did not adhere to a study regimen are also useful for designing future clinical trials and for adjusting current trials to improve adherence levels. These data will also be needed to optimize adherence levels once an effective microbicide product is introduced into the market.