Adolescent development brings many opportunities to adolescents as research participants. They may be enthusiastic participants, especially if the focus of the research is of interest to the adolescents. Their increasing sense of self and development of autonomy may yield open, thoughtful responses less likely with older research participants. At the same time, the researcher must be aware of particular vulnerabilities of adolescents. Cognitive and reasoning capacity emerges gradually over the adolescent decade, making younger adolescents less capable than older adolescents of effective reasoning. The researcher can adjust materials to be understandable to the younger adolescent but should also recognize that inexperience could increase anxiety or emotionality about an issue not problematic to an older adolescent or adult. Making clear the right of the adolescent to refuse to discuss particular issues usually permits more honest responses, and increases the likelihood of consent to research or parts of research. Especially stressful circumstances may cause degradation of reasoning capacity suggesting that the researcher needs to assess whether the adolescent is able to make a wise decision about participation, as well as about particular responses. The researcher can usually identify ways to alleviate stress in the study situation and should take all steps necessary to obtain both informed consent and valid responses. Existing research provides clear evidence that most adolescents, at least past age 14 or 15 years, are able to function as well as adults research participants under most circumstances. With younger adolescents and under stressful situations, the researcher can find ways to minimize risk from research and increase the likelihood of valid responses.