Though web tracking and its privacy implications have received much attention in recent years, that attention has come relatively recently in the history of the web and lacks full historical context. In this paper, we present longitudinal measurements of third-party web tracking behaviors from 1996 to present (2016). Our tool, TrackingExcavator, leverages a key insight: that the Internet Archive’s Wayback Machine opens the pos- sibility for a retrospective analysis of tracking over time. We contribute an evaluation of the Wayback Machine’s view of past third-party requests, which we find is im- perfect — we evaluate its limitations and unearth lessons and strategies for overcoming them. Applying these strategies in our measurements, we discover (among other findings) that third-party tracking on the web has increased in prevalence and complexity since the first third-party tracker that we observe in 1996, and we see the spread of the most popular trackers to an increasing percentage of the most popular sites on the web. We ar- gue that an understanding of the ecosystem’s historical trends — which we provide for the first time at this scale in our work — is important to any technical and policy discussions surrounding tracking.