The report presents ongoing and completed activities on modelling and calculations of radar signatures of ships in a sea environment. It also provides a general introduction to sea clutter and modelling of sea surfaces. The work is part of long-term research within R&T on the modelling and calculation of marine radar signatures for application in areas such as signature development and assessment, duel simulations as well as for electronic warfare assessment. Modelling and calculations of the radar signatures of ships in the sea background is a large and complex area of research. In recent years, FOI has expanded its expertise in the field. FOI has studied modelling of sea surfaces and sea clutter, and developed software for generating synthetic sea surfaces at different sea states. The surfaces have been used for calculations of clutter and for qualitative studies of radar signature of targets in a background. Especially, the influence of the background model, the sea state and ship roll for grazing angles, as for the case of naval missiles, have been studied. Results of calculations show that the background has a major impact on the radar cross section of the target; calculation of targets in free space underestimate the radar cross section compared to the target in a background. The higher the sea state, the lower the radar cross section. A ships movement in the waves has a great impact on the radar cross section, mainly due to variations in the geometric surface presented toward the radar. Continued research in this area is necessary, and proposals for future work are given in the report.