Topical microbicides are products that are being developed to prevent HIV infection and other sexually transmitted diseases (STD) through topical application to the genital and rectal epithelial surfaces. This paper is an update of the clinical section of a general guidance for the development and evaluation of microbicidal products that was first published by the International Working Group on Microbicides (IWGM) in 1996. (The preclinical section of that document will be updated separately later.) All topical microbicides should be clinically evaluated in humans for safety and effectiveness. Safety studies are necessary to evaluate the potential for systemic absorption and toxicity as well as local toxic effects, such as irritation, ulceration, burning, and itching. Reported symptoms of burning and itching are relevant to future product use and acceptability. Irritation and ulceration of the vaginal, cervical, penile, or rectal epithelium have the potential to result in an increased transmission of HIV and other STD. Effectiveness studies to assess the prevention of HIV infection or STD, depending upon the product indication, are subsequently conducted. These trials need to be large enough to detect clinically meaningful levels of protection. For spermicidal microbicides, additional contraceptive effectiveness studies are also needed.