The understanding of fatigue crack closure has been proved to be a challenging and controversial topic among the fatigue community over the last three decades. Since the pioneering work of Elber, different forms of crack closure have been identified (e.g. roughness and oxide induced), however, plasticity-induced crack closure has been shown to be particularly relevant in engineering applications. The effect of the specimen (or component) thickness has been shown to have a significant effect on closure behaviour and this seems to be related to the relative size of the plastic zone. The state of stress at the crack front can be predominantly plane stress, or plane strain depending on the thickness and loading conditions. Real cracks are inherently three-dimensional; plane stress-like behaviour is found close to the region where the crack front intersects the free surface, whereas most of the crack front will experience something close to plane strain. The aim of this thesis is to investigate plasticity induced fatigue crack closure from both modelling and experimental points of view. On the modelling side combined analytical and numerical techniques and finite element analyses have been used to investigate 2D plane stress, 2D plane strain and 3D crack closure problems. The influence of different effects such as different material models, surface effects, crack front curvature and residual stresses have been investigated. The experimental part of this thesis gives particular attention to the investigation of thickness effects on the closure behaviour (both close to and remote from the surface) and on fatigue crack propagation. Fatigue crack propagation is measured optically and crack closure is assessed using traditional compliance techniques (clip gauge and back face strain gauge) and Digital Image Correlation methods. This last technique was proven to be a good alternative to gauges usually used to assess surface closure. Finally, experimental results are compared with modelling predictions in order to identify strengths and limitations of these techniques. This thesis supports that crack closure effects should be taken into account for fatigue life predictions of structures under variable amplitude loading.