Clinical research studies, both nursing and medical, occur in many critical care units. As with any clinical study, it is necessary to obtain informed consent from any person participating in the study. Although many patients are able to agree or refuse to participate in the study, many are unable to do so for a variety of reasons. These patients make up vulnerable populations or people with questionable capacity. This brief article will review patients who may be vulnerable and define the role of the critical-care nurse when caring for these patients.