This thesis is concerned with soil erosion from hillslopes in semi-arid Spain. It approaches the problem f rom three related angles which are reflected in its structure: Part 1: The literature is evaluated to determine the current conceptual understanding of the processes of soil erosion, and the organisation of those concepts and evidence from field and laboratory studies into predictive models of the major processes of erosion - rainfall dynamics, rainsplash, infiltration and runoff generation, runoff routing and wash erosion mechanics. Part 2: A field study of the major factors of erosion and their results is conducted over two field seasons for four experimental hilislope sites in southern Spain, and provides detailed information on the spatial and temporal patterns of erosion and their controls. Four main contextual variables are analysed - topography, vegetation cover, surface sediments (soil and coarse fraction) and micro-topographic roughness. The results from four process experiments are presented - infiltration from ponded and simulated rainfall experiments; runoff generation and soil loss from fifty microplot rainfall simulations; natural storm runoff and erosion losses from sub-site plots situated at the midpoint and base of each site; and seasonal and annual patterns of ground surface change derived from soil pin data. Part 3: Concern is with the development of selected erosion process simulation models. These include - the stochastic generation of a temporally nested rainfall series (yield, intensity, duration of wet and dry periods) for the annual/seasonal distribution of daily rainfall, and, subject to threshold criteria, the hourly pattern of storms within a day; an infiltration-runoff generation model based on a modified Green and Ampt (1911) infiltration equation, which predicts time to runoff, volume of pre-saturation soil storage, and runoff volumes. The latter are routed across plots according to an algorithm which determines potential flow lines in three dimensional space. A finite difference kinematic flow model using the Manning flow equation is developed and calibrated from microplot data; finally an erosion model developed from the prototype of Meyer and Wischmeier (1969) predicts spatial and temporal variability of erosion from hillslopes. Major changes to the original model include three-dimensional space and contour curvature, dynamic modelling of flow generation and routing, and physically based detachment and transport mechanics of overland flow. The thesis is divided into two volumes - Volume I comprises the text arranged into three parts. Volume II, for ease of reference, presents supporting and background material, which may be read in parallel with Volume I, and includes Appendices of raw data, results of data analysis, computer programs and simulation results; a list of symbols by chapter; a set of photographic plates, and a full bibliography.