This research sought to determine whether the implementation of Housing First in a large-scale, multi-site Canadian project for homeless participants with mental illness shows high fidelity to the Pathways Housing First model, and what factors help or hinder implementation. Fidelity ratings for 10 Housing First programs in five cities were made by an external quality assurance team along five key dimensions of Housing First based on 84 key informant interviews, 10 consumer focus groups, and 100 chart reviews. An additional 72 key informant interviews and 35 focus groups yielded qualitative data on factors that helped or hindered implementation.