In this paper, we present a semi-automatic approach for summarizing the content of large execution traces. Similar to text summarization, where abstracts can be extracted from large documents, the aim of trace summarization is to take an execution trace as input and return a summary of its main content as output. The resulting summary can then be converted into a UML sequence diagram and used by software engineers to understand the main behavioural aspects of the system. Our approach to trace summarization is based on the removal of implementation details such as utilities from execution traces. To achieve our goal, we have developed a metric based on fan-in and fan-out to rank the system components according to whether they implement key system concepts or they are mere implementation details. We applied our approach to a trace generated from an object-oriented system called Weka that initially contains 97413 method calls. We succeeded to extract a summary from this trace that contains 453 calls. According to the developers of the Weka system, the resulting summary is an adequate high-level representation of the main interactions of the traced scenario.