This book is a concise summary of the present knowledge about skeletal muscle fibres. The fibre types were characterized from different points of view. The difficulties and possibilities of classifying muscle fibres in distinct non-overlapping types were shown. A main emphasis is put on metabolic fibre typing by cytophotometry as well as the adaptability of a given fibre type to altered physiological and pathological conditions. Extensive analyses of several rat hindlimb muscles revealed regional differences of fibre properties within the muscles and showed the influence of ageing, myopathy, hypoxia, diabetes and Ginkgo biloba-treatment on the different fibre types. The plasticity of muscle fibres was demonstrated by the flexibility of fibre metabolism as an adaptive mechanism.