The III-nitride semiconductor materials system is used in thin-film-based optoelectronic devices. GaN and InGaN in particular have been used to produce efficient blue and green light emitting diodes. However, III-nitride thin films typically contain very high densities of dislocations, a line defect known to negatively affect device performance and lifetimes, but despite this the performance of III-nitride-based devices is much less affected by dislocations than devices based on other semiconducting systems. These dislocations have been the subject of extensive study but experimental and theoretical reports still present conflicting data. In particular, dislocations do not exist in isolation in III-nitride thin films, but can interact with point defects, dopants and alloying elements, and may induce local compositional segregation in III-nitride alloy epilayers. This thesis presents a study of the structure and properties of dislocations in GaN and InGaN and of their interactions with point defects, dopants and alloying elements. Specifically, a new theoretical analysis of a common extended dislocation core in GaN is presented and the conclusions are verified by experimental data. Random alloy microstructures are predicted and analysed for InGaN quantum wells of different thicknesses, followed by a theoretical and experimental study of the variation in InGaN alloy composition around dislocation cores. Finally, the mechanisms by which dislocation cores can act as preferential diffusion pathways for native point defects are studied, providing insight into possible mechanisms by which compositional segregation or dislocation movement could occur. The methods presented here have a broad relevance beyond the III-nitride materials system and highlight the importance of assessing the interactions of dislocations with other defects, to achieve a better understanding of dislocation properties and their influence on device performance.