Hereditary spastic paraplegia is a rare disorder with gait disturbance due to a degeneration of the corticospinal tract, sometimes accompanied by involvement of other systems. Out of the 20 loci known so far, eight genes have now been identified, allowing the first molecular and cell studies in the pathophysiology of the disorder. These should also help to understand the function of the corticospinal tract at the molecular level and design strategies to prevent and treat spasticity due to more common causes. The proteins encoded by these genes play a role in development, in signal transduction between axons and myelinating cells, in cellular, particularly axonal trafficking or in energy metabolism. Some of them have actions in several areas of cellular function. Here we review the present knowledge about the genes involved in hereditary spastic paraplegia, a field presently undergoing rapid change.