Examine the challenges of four service delivery models (i.e., fixed, mobile, satellite and home visits) and how service delivery may impact on NEP HIV prevention efforts.Using a modified ethnographic approach, semi-structured interviews concerning policies and procedures were conducted with staff (n = 59) of NEPs (n = 15) in Ontario. An iterative, inductive analytic process was used.According to workers and managers, effectiveness of NEP prevention efforts depend on client development and retention and service design. Fixed and satellite sites, home visits and mobile services provide varied levels of temporal and spatial accessibility. Combining modes of delivery can offset the disadvantages of individual modes.NEP evaluations that do not consider service and resource factors run the risk of concluding that NEPs are ineffective when it may be that the program works for a small proportion of IDUs whom the NEP has the resources to serve.