Definitions of maltreatment exist with utility in various settings—legal, state agency, policy, research, and treatment. This chapter looks at how ethical considerations utilize a specifically defined event as maltreatment and how this affects the research process and the research participant, which in turn may be affected by specific contexts. In addition to reviewing current legal standards around enrolling vulnerable research subjects, this chapter also explores emerging issues and how the ethical dilemmas have changed as child maltreatment research has become more sophisticated. To illustrate these changes, examples of contemporary child abuse and neglect research projects that have addressed the new ethical challenges in measurement development, data collection, data handling, and analysis are provided. Last, close attention is paid to the complexities faced by researchers who directly study children residing in foster care who, because of the circumstances engendering their entry into substitute care, constitute a particularly vulnerable research population.