This chapter concerns concepts of adaption in a binaural audio context (i.e. headphone-based three-dimensional audio rendering and associated spatial hearing aspects), considering first the adaptation of the rendering system to the acoustic and perceptual properties of the user, and second the adaptation of the user to the rendering quality of the system. We start with an overview of the basic mechanisms of human sound source localisation, introducing expressions such as localisation cues and interaural differences, and the concept of the Head-Related Transfer Function (HRTF), which is the basis of most 3D spatialisation systems in VR. The chapter then moves to more complex concepts and processes, such as HRTF selection (system-to-user adaptation) and HRTF accommodation (user-to-system adaptation). State-of-the-art HRTF modelling and selection methods are presented, looking at various approaches and at how these have been evaluated. Similarly, the process of HRTF accommodation is detailed, with a case study employed as an example. Finally, the potential of these two approaches are discussed, considering their combined use in a practical context, as well as introducing a few open challenges for future research.