This paper focuses on the clients? impact on estimation accuracy in software development projects. Client related factors contributing to effort overruns as well as factors preventing overruns are investigated. Based on a literature review and a survey of 300 software professionals we fi nd that: 1) Software professionals perceive that clients impact estimation accuracy. Changed and new requirements are perceived as the clients? most frequent contribution to overruns, while overruns are prevented by the availability of competent clients and capable decision makers. 2) Survey results should not be used in estimation accuracy improvement initiatives without further analysis. Surveys typically identify directly observable and project specifi c causes for overruns, while substantial improvement is only possible when the underlying causes are understood.