A well designed event trace data format is the basis of all trace-based analysis methods. In this paper, we introduce the Open Trace Format Version 2 (OTF2). It is a major re-design based on the experiences of its predecessor formats, the Open Trace Format (Version 1) and the EPILOG trace format. It comes with a new file encoding, a close integration in the trace recording system Score-P, and a number of improvements for performance and scalability. Besides the actual format, it consists of a read/write support library with a powerful API plus supportive tools, which are distributed as Open Source software. Furthermore, OTF2 will serve as a joint data source for the analysis tools Scalasca and Vampir in the near future.