While Istio offers complete service mesh capabilities out of the box, organizations often need to extend their functionalities to meet specific requirements. This chapter explores how to extend Istio with WebAssembly plugins, with a focus on the practical elements of creating, deploying, and managing these extensions. We'll also look at Istio's extensibility points using WebAssembly, with a primary focus on C++ examples since it is the most mature and well-documented language for Istio extensions. While Rust and AssemblyScript are also supported for WebAssembly development, we'll stick with C++ code samples to provide clear, actionable guidance. Given that this book is intended for DevOps engineers and platform architects who are already aware with Istio's key concepts, we will focus on practical implementation details with relevant code snippets rather than in-depth theoretical discussions, giving you the hands-on knowledge you need to tailor Istio to your organization's specific requirements.