The work reported here is in the educational domain and deals specifically with the observation of instrumented pedagogical activities. Our objective is to observe learner behaviour within a web-based learning environment and to understand it. In some cases, this understanding can lead to improve the learning scenario itself. In this article, we first show how to gather observations from various sources. Next, we propose to compose these observations into a trace. This trace reflects the activity of the learner and will be used as a basis for interpreting his/her behaviour. We have illustrated our approach through an experiment carried out in a dedicated web-based learning environment.