Epidemiologists are often asked to evaluate product safety or provide information about the history of disease and use of health services. Most of this research is conducted through sponsorship from a governmental or other non-profit agency, or with direct support from a for-profit company with an economic interest in the outcome. Vast amounts of information that are routinely collected for administrative and billing purposes may also be used for research. These data can reveal information about the etiology of disease, utilization patterns of prescription drugs, and trends in disease occurrence. Using third-party data requires a trusting partnership between researchers and data custodians. Although such data offer public health benefits, their use can also lead to embarrassment and legal action. Five guiding principles will help outside contractors facilitate access to third-party data and avoid pitfalls. (1) Understand the sponsor's objectives by understanding the purpose of the research. (2) Identify and approach data resources that have appropriate information. (3) Consider special issues relating to accessing confidential information. (4) Establish terms of the research engagement with the sponsor. (5) Establish ground rules with the data provider.