Persons who are homeless and mentally ill present unique challenges to service providers and human service systems. In vivo case management approaches such as assertive community treatment (ACT) have shown promise in engaging this population. This paper explores case management models employed within the ACCESS program, a five year, 18-site demonstration program enriching services for homeless persons with serious mental illness. We describe the implementation of case management with ACCESS programs and determine the extent of variation across sites using a measure of fidelity to ACT. While programs reported using four models, much similarity was found among programs on multiple dimensions.