Based on calls for studying effects and effectiveness of urban sustainability transition labs" (USTLs), this chapter explores in how far the six transition projects in Phoenix allowed for learning that is likely to yield lasting sustainability outcomes in the future. It presents a general evaluative scheme for USTLs and a cautious interpretation of exemplary evaluation results (USTL Phoenix). Eventually, the study intends to support people keen on running USTLs who want to learn from previous experiences. More generally, the chapter expands the set of case studies on urban sustainability transition initiatives that steadily broadens and enhances the base for evidence-supported urban sustainability transition efforts. The chapter structures the key features of USTLs according to an evaluative logical model with inputs, process, outputs, and outcomes. The USTL in Phoenix has developed a promising pathway to making contributions to the sustainability transition in Phoenix.