Both hereditary and environmental factors are important in the interactive growth of the ocular tissues responsible for determining the refractive state of the eye. Myopia has a low prevalence in otherwise healthy children and is seen with an increased prevalence early in life in many systemic and ocular diseases. Predicting how refractive error will change in any individual child after birth or at any stage of ocular development is not possible at present, although, trends can be seen in longitudinal studies. In the disease states associated with an increased prevalence of myopia, information regarding the time of onset of myopia and the specific values for the refractive components, is lacking, so that underlying mechanisms of myopia development and whether the myopia is congenital or developmental, are not known. In adults, three different types of myopia can be characterised on the basis of clinical characteristics which have prognostic significance for ocular disease. The mechanisms of ocular growth that lead to these forms of myopia are not clear. At present, treatments to prevent or slow myopic progression have been marginally beneficial and of questionable value.