Islands Architecture is a web application design pattern in which the majority of a page is rendered as static, non-interactive HTML and only specific interactive components — “islands” — are hydrated and made interactive in the browser with JavaScript. The term was coined by Etsy’s Katie Sylor-Miller and popularised by Jason Miller in a 2020 blog post. The pattern directly addresses the performance cost of shipping large JavaScript bundles for pages that are mostly static content with isolated interactive areas such as a header, a carousel, or a comment form. Each island hydrates independently, allowing critical islands to load first while less important ones load lazily. Astro, released in 2021, is the most prominent framework built around Islands Architecture, implementing it through its component directives (client:load, client:visible, client:idle). Frameworks such as Fresh (Deno) and Îles also adopt this model. Islands Architecture typically yields significant improvements in Total Blocking Time and INP compared to traditional single-page application hydration approaches.