Fusion: The Performance of Architecture explores the work of award-winning, Boston-based architecture firm Payette, a leader in the design of complex settings for science and healthcare. Payette's work embodies the integration of design and performance that is essential to the creation of humane and sustainable buildings of any type. To achieve this integration amidst the programmatic intricacy, technological complexity, and intense energy use of hospitals and laboratories, the firm draws on its almost ninety-year history of progressive innovation. It draws, as well, on an inclusive, collaborative, research-oriented culture that is a model for the profession. Fusion presents Payette's philosophy and traces the firm's contributions through concise histories of laboratory and hospital design. It explores the core principles that underlie its work - Identity and Transformation, Materiality and Craft, Taming Complexity and Measuring Performance - and digs deeply into seven of the firm's most recent projects. Other chapters describe the process of nurturing the design excellence and practice culture that earned Payette the 2019 AIA Architecture Firm Award. The monograph's 400 diagrams, drawings, and photographs reveal the firm's principles and methods, along with the open-source tools it has developed to enable it to design, not "by the numbers," but with the numbers. A gallery of architectural "fingerprints" presents plan views of more than 100 of Payette's projects, drawn to a common scale. With a preface by Z Smith, Director of Sustainability and Building Performance at EskewDumezRipple, and an introduction from Kevin Sullivan, President of Payette, Fusion includes essays by Sullivan and partners James Collins, George Marsh, Leon Drachman, Andrea Love and Peter Vieira, as well as a critical reflection by Mark Lee, Chair of the Department of Architecture at the Harvard Graduate School of Design.