The causes and effects of hot-spot heating, an important failure mode of photovoltaic arrays, are investigated. A computer program, for modelling the electrical processes leading to hot-spot heating in networks of solar cells, is presented. A variety of techniques (infra-red imaging, scanning electron microscopy, and electrical measurements) are used to identify the microscopic processes causing non-uniform power dissipation in reverse biased, commercial silicon solar cells. A computer controlled laser scanning system, used to investigate the uniformity of solar cells, is described.