Blazor has grown, and with that growth comes a simple question: How should we build Blazor apps today? This book answers that question by building a real application step by step. We start with what Blazor is, why it is not just WebAssembly, and how the different hosting models fit together. From there, we create components, manage state, build forms with validation, add APIs, secure the app with authentication and authorization, use JavaScript when it makes sense, and test our components with bUnit. We also look at the key aspects of modern Blazor development, including render modes, server-side rendering, WebAssembly, Aspire, OpenTelemetry, debugging, deployment, and how to work with existing sites when starting from scratch is not an option. The goal is not only to copy code but also to understand why we choose one approach over another. Should this be SSR, Server, WebAssembly, or Auto? Where should interactivity live? What changes when the code runs in the browser? We answer those questions without making things more complicated than they need to be. Whether you're new to Blazor or upgrading from an earlier edition, the fourth edition brings the book up to date with .NET 10, Aspire, tracing, metrics, testing, and modern Blazor app development. Own a raccoon cover already? The collection must continue. The raccoons insist.