This chapter explores the challenges in prioritizing research involving hidden or marginalized populations, in recruiting these subpopulations to participate in research, in the measurement in studies utilizing non-traditional sampling techniques, and in the retention of these populations during the span of a study. The population of the United States is increasingly diverse, and this trend is expected to continue well into the future. Funding is of paramount importance to the inclusion of hidden and underserved populations in research. Regardless of the sampling method utilized, there are several steps researchers can take to encourage participation by members of marginalized or hidden populations. When the implications of research targeting hidden or underserved populations are being considered, ethical concerns can naturally arise. Use of ethical, detailed research goals and continuous consent protocols ensures that participants from hidden or underserved populations feel safe participating over the duration of the study.