This case study presented the requirements engineering process in general and related to three well-known software engineering processes, life cycle model, Rational Unified Process and Scrum process. This thesis elaborated the phases of requirements engineering process and focused on one part of it, requirements presentation. A set of commonly used requirements presentation forms, such as use case diagram, use case, sequence diagram and state diagram, were presented and compared on the basis generality and formality. After that, software maintenance was introduced. The study then described the context of software maintenance and different types of maintenance tasks. In addition, three different maintenance processes were studied: quick-fix model, iterative-enhancement model and the IEEE 1219-1998 standard for software maintenance. After presenting the software maintenance process, general challenges related to software maintenance and the exploitation of documentation in maintenance were presented. This thesis studied five different software under maintenance. The characteristics of these software varied, they used different maintenance process and exploited different kind of documentation. The most commonly used documents were use cases and user interface descriptions at different levels. The most important function of documents was to serve as a starting point for maintenance tasks. They were also used in learning a new software and as a source of information. Some issues were found that complicated the exploitation of documents. Problems in maintenance had been caused by e.g. incomplete documentation, complete lack of documentation and documents that are hard to understand or of poor quality. Correcting these problems would not require miracles, but they could be overcome with a systematic approach and good judgement.