This paper aims to provide a basis for the improvement of software estimation research through a systematic review of previous work. The review identifies 304 software cost estimation papers in 76 journals and classifies the papers according to research topic, estimation approach, research approach, study context and data set. Based on the review, we provide recommendations for future software cost estimation research: 1) Increase the breadth of the search for relevant studies, 2) Search manually for relevant papers within a carefully selected set of journals when completeness is essential, 3) Conduct more research on basic software cost estimation topics, 4) Conduct more studies of software cost estimation in real-life settings, 5) Conduct more studies on estimation methods commonly used by the software industry, and, 6) Conduct fewer studies that evaluate methods based on arbitrarily chosen data sets.