Process event data is a fundamental building block for process mining as event logs portray the execution trails of business processes from which knowledge and insights can be extracted. In this Chapter, we discuss the core structure of event logs, in particular the three main requirements in the form of the presence of case IDs, activity labels, and timestamps. Moreover, we introduce fundamental concepts of event log processing and preparation, including data sources, extraction, correlation and abstraction techniques. The chapter is concluded with an imperative section on data quality, arguably the most important determinant of process mining project success.