This monograph introduces tracking on the web to readers with little or no previous knowledge of the topic. Tracking is the collection of data about an individual's activity in multiple contexts and the retention, use, or sharing of data derived from that activity outside the context in which it occurred. This work covers the topic primarily from the perspective of computer science and human-computer interaction but also includes relevant law and policy aspects. It primarily focuses on tracking as a near-ubiquitous commercial practice that emerged through a symbiotic relationship with websites, mobile apps, and other internet-based services. It aims to provide an overarching narrative spanning this large research space. The monograph starts by introducing the concept of tracking, and provides a short history of the major developments of tracking on the web. It presents research covering the detection, measurement and analysis of web tracking technologies, and delves into the countermeasures against web tracking as well as studies into end-user perspectives on tracking. The work also focuses on tracking on smart devices including smartphones and the Internet of Things, and concludes with emerging issues affecting the future of tracking across these different platforms.