Inertia.js is a library that simplifies the development of single-page applications (SPAs) by combining server-side and client-side rendering, allowing developers to utilize server-side frameworks like Laravel, Ruby on Rails, or Django without the need for separate REST or GraphQL APIs. It addresses common challenges in building SPAs, such as state management, routing, and browser issues, by leveraging AJAX calls and providing built-in solutions like NProgress.js for loading indicators. Inertia is framework-agnostic and works with both server-side and client-side frameworks, including official adapters for Rails and Laravel on the backend, and React, Vue.js, and Svelte on the frontend. It is designed to maintain a tight coupling between server-side controllers and views while enabling modern client-side framework usage, making it particularly suitable for monolith applications and dashboards, though it may not be ideal for SEO-driven websites or multi-client support scenarios. Despite not supporting server-side rendering, tools are available to pre-render and cache static HTML versions of Inertia websites.