This thesis deals with the assessment of the strength and durability of steel to composite joints for composite superstructures on ships where reduced weight is a design driver. The purpose of the work is to understand the long-term performance characteristics of hybrid connections to allow for improvements to the design of hybrid structures. Two joints were investigated in the present research. The first was a full-scale connection suitable for application in superstructures of marine vehicles, specifically a helicopter hanger on a naval vessel. The second was a generic steel/composite connection for testing performance after hygrothermal ageing. The strength and durability of the full-scale connection were examined in compression, the loading scenario representative of in-service conditions. The results indicated that the static and fatigue performance were in excess of the realistic in-service loading conditions. Failure for both static and fatigue tests were comparable and therefore good confidence in the prediction of the joint’s failure was achieved. The generic hybrid connection was artificially aged through immersion in water. The results indicated that there was no significant reduction in the performance of the joint in either static tension or bending. The numerical modelling highlighted a number of issues. Due to the geometry of the joint high stress concentration factors were observed in some locations. It is in these areas that failure of the joint was predicted in the numerical modelling. Similar results were obtained experimentally and this gave confidence in the modelling of the joint. Numerical parametric and optimisation studies were conducted to assess the influence of the joint geometry on performance characteristics obtained from both the experimental and numerical studies. This highlighted that improvements to the performance of the joint could be obtained through geometric changes alone.