This document provides an introduction to continuous integration (CI). It discusses how CI automates the manual processes of building, testing, and deploying code. The document explains that CI aims to keep the main development branch always green by quickly identifying and fixing any issues. It describes how a CI server works by monitoring code repositories, running jobs on agents to build, test and deploy code, and notifying developers of failures. Finally, it compares open-source CI tools like Jenkins in terms of their pros and cons.