Program comprehension is a key activity in software engineering and plays an important role in software development, software maintenance, and software reuse. Since the advent of software engineering, program comprehension has always been a hot research hotspot issue in this field. With the increasing complexity and popularity of software, the needs for program comprehension have been changed. The program self-understanding and self-awareness have gradually become new focuses. Therefore, it is highly desired to re-examine the purposes, the tasks and the techniques of program comprehension. Firstly, this paper dicsusses the program comprehension from the 3 perspectives, namely, the engineering, the learning cognition, as well as the techniques. Then, it shows the degree of research attentions through literature analysis. Furthermore, it discusses the research progress from three aspects, i.e., the cognitive process, the methods and techniques, and the software engineering tasks. Finally, it discusses the development trend and challengs.