This study is a formative evaluation of the OneStop Career Center Integrated Service Delivery (ISD) Model within the California Workforce System. The study was sponsored by the California Workforce Investment Board. The study completed four in-depth case studies of California OneStops to describe how they implemented the ISD model which brings together local Workforce Investment Act (WIA) program with state program operated by California's Employment Development Department to deliver services to job seekers. The four sites included urban, suburban and rural sites from different regions of the state. Some sites were operated by private entities and others run by government agencies. The case analysis included a description of why local areas adopted the new model, how they implemented the model and how managers and staff perceive the model's success. In addition a cost analysis was done of each site. The study identified a series of "critical success factors" that help or hinder the success of ISD implementation and suggest strategies that can help other OneStops implement ISD. The critical success factors (CSFs) identified fall into two categories; the OneStop's context and the implementation process. Within the two categories five critical success factors were identified. Contextual critical success factors were: organizational culture and political culture, along with the local and regional Context. Within the implementation process three factors emerged: the management structure, the decision-making process, and the formal and informal implementation strategies. The report concludes with implications for expanding the model to additional sites and ideas for a summative evaluation. An appendix includes: ISD Case Study Protocol, ISD Staff Interview Guide, ISD Site Administrators Interview Guide, CVRural - OneStop Layout, ALPHA OneStop Layout, and ALPHA Workshop Handout. (Contains 25 footnotes, 6 text boxes, 2 flowcharts, 38 figures, and 22 tables.) [This report was prepared for the California Workforce Investment Board.]