Program and system comprehension are vital parts of the software maintenance process. We discuss the need for both perspectives and describe two methods that may be integrated to provide a smooth transition in understanding from the system level to the program level. Results from a qualitative survey of expert industrial software maintainers, their information needs and requirements when comprehending software are initially presented. We then review existing software tools which facilitate system level and program comprehension. Two successful methods from the fields of data mining and concept assignment are discussed, each addressing some of these requirements. We also describe how these methods can be coupled to produce a broader software comprehension method which partly satisfies all the requirements. Future directions including the closer integration of the techniques are also identified.