The body of literature exploring neighborhood effects on health has increased rapidly in recent years, yet a number of methodologic concerns remain, including preferred methods for identification and delineation of study neighborhoods. In research combining census or other publicly available data with surveys of residents and/or street-level observations, questions regarding neighborhood definition take on added significance. Neighborhoods must be identified and delineated in such a way as to optimize quality and availability of data from each of these sources. IMPACT (Inner-City Mental Health Study Predicting HIV/AIDS, Club and Other Drug Transitions), a multilevel study examining associations among features of the urban environment and mental health, drug use, and sexual behavior, utilized a multistep neighborhood definition process including development of census block group maps, review of land use and census tract data, and field visits and observation in each of the targeted communities. Field observations were guided by a preidentified list of environmental features focused on the potential for recruitment (e.g., pedestrian volume), characteristics commonly used to define neighborhood boundaries (e.g., obstructions to pedestrian traffic, changes in land use), and characteristics that have been associated in the literature with health behaviors and health outcomes (such as housing type and maintenance and use of open spaces). This process, implemented in February through July 2005, proved feasible and offered the opportunity to identify neighborhoods appropriate to study objectives and to collect descriptive information that can be used as a context for understanding study results.