Axonal transport is the process whereby motor proteins actively navigate microtubules to deliver diverse cargoes, such as organelles, from one end of the axon to the other, and is widely regarded as essential for nerve development, function and survival. Mutations in genes encoding key components of the transport machinery, including motor proteins, motor adaptors and microtubules, have been discovered to cause neurological disease. Moreover, disruptions in axonal cargo trafficking have been extensively reported across a wide range of nervous system disorders. However, whether these impairments have a major causative role in, are contributing to or are simply a consequence of neuronal degeneration remains unclear. Therefore, the fundamental relevance of defective trafficking along axons to nerve dysfunction and pathology is often debated. In this article, we review the latest evidence emerging from human and in vivo studies on whether perturbations in axonal transport are indeed integral to the pathogenesis of neurological disease.