Amyotrophic lateral sclerosis (ALS) was first described by Charcot in 1869 as what we would now call a sporadic disease-a disease believed to occur without a strong genetic influence. Only within the past 10 years has it been possible to fully explore genetic influence on disorders that seem to occur sporadically but likely result from the convergence of multiple genetic and environmental factors. This article reviews the genetics of familial ALS and summarizes current investigations of genetic influence in sporadic ALS. Genetic study clearly offers the potential for identification of molecular targets that would allow development of rational therapies for various forms of ALS, but much work remains.