To improve the software development process is named by both the European Union and the United States government as an important task for society. The constant problem with effort overruns and estimation inaccuracy is a main part of the software development problem. Empirical research on software effort estimation is a key part of the continuing effort by researchers and practitioners to improve the way in which software development projects are carried out.As part of this effort, a study on eighteen of the latest projects at a Norwegian software consultancy was done. The study was done by interviewing the project managers responsible for the projects, having them provide key project data, and their assessments of different project properties related to effort estimation. The study focused on answering research questions related to:• The effect the contractor-customer relationship and customer properties have on estimation accuracy• The effect utilizing experience data has on estimation accuracy• The role of estimation accuracy when assessing software project successThe analysis of the collected empirical data showed that reduced effort overruns was associated with increased contact frequency with the customer and contracts that share the risk between contractor and customer.Utilization of experience data, and the use of checklists, was also found to have a positive impact on estimation accuracy.There was not found any strong correlation between project managers’ project success assessment and estimation accuracy, indicating that estimation accuracy and project manager success assessment contribute with two different, but important viewpoints when software project success is to be assessed.In addition to the empirical study and its results, the thesis presents a review of existing group combination techniques for software effort estimation. The review was motivated by recent studies that have suggested that to do software estimation as a group is beneficial. The review presents techniques that vary largely as to how they structure the interaction among the group members, and how their opinions are aggregated. A thorough discussion on the argumentation behind the techniques, and the consequences they have is given in the review.The empirical data collected during the work with this thesis suggests different ways in which software contractors could improve their estimation ability and reduce their effort overruns.The conclusions of this thesis is, that to increase estimation accuracy, software contractors should: (i) involve the customer, and nurture the customer relationship, (ii) add some repeatable structure to the estimation process, but be careful not to add too much structure, (iii) gather and utilize experience data in the estimation process and (iv) evaluate projects when they are done. In doing the evaluation both objective data on effort, schedule and functionality compliance and subjective assessments of project success from key stakeholders, as customer, user, project manager, developers and management should be gathered.