Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease predominantly affecting upper and lower motor neurons. The disease leads to relentlessly progressive weakness of voluntary muscles, with death typically resulting from diaphragmatic failure within 2-5 years. Since the discovery of mutations in SOD1, which account for ∼2% of ALS cases, increasing efforts have been made to understand the genetic component of ALS risk, with the expectation that this insight will not only aid diagnosis and classification, but also guide personalized treatment and reveal the mechanisms that cause motor neuron death. In this Review, we outline previous and current efforts to characterize genes that are associated with ALS, describe current knowledge about the genetic architecture of ALS - including the relevance of family history - and the probable nature of future gene discoveries, and explore how our understanding of ALS genetics affects present and future clinical decisions. We observe that many gene variants associated with ALS have effect sizes between those of mutations that greatly increase risk and those of common variants that have a small effect on risk, and combine this observation with insights from next-generation sequencing to explore the implications for genetic counselling.