This paper summarizes estimation knowledge through a review of surveys on software effort estimation. Main findings were that: (1) Most projects (60-80%) encounter effort and/or schedule overruns. The overruns, however, seem to be lower than the overruns reported by some consultancy companies. For example, Standish Group's 'Chaos Report' describes an average cost overrun of 89%, which is much higher than the average overruns found in other surveys, i.e., 30-40%. (2) The estimation methods in most frequent use are expert judgment-based. A possible reason for the frequent use of expert judgment is that there is no evidence that formal estimation models lead to more accurate estimates. (3) There is a lack of surveys including extensive analyses of the reasons for effort and schedule overruns.