This chapter discusses types of data collection instruments and how to evaluate their precision. Because collected data always include both accurate information and measurement error, the best tool is the one that measures a variable of interest most directly, accurately, and consistently. This is true for both quantitative and qualitative measures of participant demographics and study variables. Thus, chapter content describes the uses, advantages, and disadvantages of three instrument types: Self-report, observational, and biophysical. Additionally, statistical criteria for evaluating different types of validity and reliability of quantitative tools are outlined, while study dependability, confirmability, and credibility are presented as criteria for evaluating the accuracy of researcher-as-instrument in qualitative research. Recommendations for how to find research instruments and improve participant response rates are included.