Spec is the default UI building framework for Pharo. With Spec, developers focus on user interaction and widget layout. Spec is built around the Model View Presenter pattern. Presenters are responsible for defining the interaction logic of the application. They enforce the interaction of their subelements as well as domain objects. With Spec, existing presenters or even complete UIs are reused and configured to form a new user interface. Spec is widget framework (GTK, Morphic, Toplo) agnostic. It makes desktop applications simple to build. This book shows the basics of Spec in Pharo 12 and how it enables reuse of user interface elements. It focuses on Spec 2.0 features such as new widget layouts, dynamic widget building and more advanced features such as transmissions or commands. The book contains: - Deep explanations of various Spec 2.0 facets. - A tutorial gets you started in less than 20 minutes. - A minimal email client revisiting all Spec 2.0 features.