Over the last decades, the complexity of high-tech systems, and the softwaresystems controlling them,has increased considerably. In practice, it is hard to keep knowledge and documentation of these ever-evolving software systems up-to-date with their actual realization; we are dealing with legacy software. Clearly, this lack of knowledge, insight, and understanding is more and more becoming a critical issue. Processmining provides an interesting opportunity to improve understanding and analyze software behavior based on observations from the system on the run. However, a concrete software process analysis methodology was lacking. This paper 1) discusses a software process analysis case study at ASML, a large high-tech company, and, based on the lessons learned, 2) presents a concrete methodology for analyzing software processes. The presented methodology actively includes the system under analysis and is based on practical experiences in applying process mining on industrial-scale legacy software. Keywords-Reverse Engineering; Process Mining; Methodology; Case Study; Passive Learning; Performance Analysis; Program Analysis; Model-Driven Analysis.