Software maintenance and evolution of systems was first addressed by Meir M. Lehman in 1969. Over a period of twenty years, his research led to the formulation of Lehman’s Laws (Lehman 1997). Key findings of his research conclude that maintenance is really evolutionary development and that maintenance decisions are aided by understanding what happens to systems (and software) over time. Lehman demonstrated that systems continue to evolve over time. As they evolve, they grow more complex unless some action such as code refactoring is taken to reduce the complexity.
In the late 1970s, a famous and widely cited survey study by Lientz and Swanson, exposed the very high fraction of life-cycle costs that were being expended on maintenance. They categorized maintenance activities into four classes:
- Adaptive – modifying the system to cope with changes in the software environment (DBMS, OS) 
- Perfective – implementing new or changed user requirements which concern functional enhancements to the software
- Corrective – diagnosing and fixing errors, possibly ones found by users 
- Preventive – increasing software maintainability or reliability to prevent problems in the future 
The survey showed that around 75% of the maintenance effort was on the first two types, and error correction consumed about 21%. Many subsequent studies suggest a similar problem magnitude. Studies show that contribution of end users is crucial during the new requirement data gathering and analysis. This is the main cause of any problem during software evolution and maintenance. Software maintenance is important because it consumes a large part of the overall lifecycle costs and also the inability to change software quickly and reliably means that business opportunities are lost.