Individuals and groups leave evidence of their lives when they are engaged in their activities. In this way, they create a rich amount of material that tells us about their behaviours, opinions and values. This material is not created for research purposes and is different from that solicited by researchers. In recent decades, the spread of new communication technologies has amplified the possibility of creating and disseminating this kind of data outside the research context.In this chapter, what people leave behind (WPLB) online is studied from a strictly methodological point of view. What kind of evidence are researchers dealing with? Is it possible to reconnect it with the traditional methodological framework? We suggest that data left behind by people and groups on the Internet should be divided into three different categories: online found data (digital traces), online retrieved data (web-mediated documents) and online captured data (online behaviours). The phase of contextualization proves essential in understanding the very nature of (online) data.This work leads to rediscovering the potential of classical methodological tools such as simple observation, documentary analysis and trace analysis. These practices provide methodological value to research projects that analyse WPLB in physical and web-mediated environments.