Research data collection that involves human subjects is dependent upon either voluntary or involuntary participation of persons. In the opinion research field, when utilizing conventional survey and opinion research methods, the degree and completeness to which individuals willingly engage in the data collection process is known as “respondent cooperation.” Respondent cooperation has been a preeminent concern of survey and opinion researchers since at least the latter part of the twentieth century both because of its implications for data quality, as well as its reflection upon research methods and the resulting ethical and regulatory considerations. In this chapter, respondent cooperation is defined and explained in detail. Specific attention is given to the importance of respondent cooperation in research, theories, and evidence of how and why respondent cooperation has changed over time, and the general profile of survey respondents.