Background: Despite the high incidence of muscle weakness in individuals with amyotrophic lateral sclerosis (ALS) or motor neuron disease (MND), the effects of exercise in this population are not well understood. This is an update of a review first published in 2008. Objectives: To systematically review randomised and quasi-randomised studies of exercise for people with ALS or MND. Search methods: We searched The Cochrane Neuromuscular Disease Group Specialized Register (2 July 2012), CENTRAL (2012, Issue 6 in The Cochrane Library), MEDLINE (January 1966 to June 2012), EMBASE (January 1980 to June 2012), AMED (January 1985 to June 2012), CINAHL Plus (January 1938 to June 2012), LILACS (January 1982 to June 2012), Ovid HealthSTAR (January 1975 to December 2012). We also searched ProQuest Dissertations & Theses A&I (2007 to 2012), inspected the reference lists of all papers selected for review and contacted authors with expertise in the field. Selection criteria: We included randomised or quasi-randomised controlled trials of people with a diagnosis of definite, probable, probable with laboratory support, or possible ALS, as defined by the El Escorial criteria. We included progressive resistance or strengthening exercise, and endurance or aerobic exercise. The control condition was no exercise or standard rehabilitation management. Our primary outcome measure was improvement in functional ability, decrease in disability or reduction in rate of decline as measured by a validated outcome tool at three months. Our secondary outcome measures were improvement in psychological status or quality of life, decrease in fatigue, increase in, or reduction in rate of decline of muscle strength (strengthening or resistance studies), increase in, or reduction in rate of decline of aerobic endurance (aerobic or endurance studies) at three months and frequency of adverse effects. We did not exclude studies on the basis of measurement of outcomes. Data collection and analysis: Two review authors independently assessed trial quality and extracted the data. We collected adverse event data from included trials. The review authors contacted the authors of the included studies to obtain information not available in the published articles. Main results: We identified two randomised controlled trials that met our inclusion criteria, and we found no new trials when we updated the searches in 2012. The first, a study with overall unclear risk of bias, examined the effects of a twice-daily exercise program of moderate load endurance exercise versus "usual activities" in 25 people with ALS. The second, a study with overall low risk of bias, examined the effects of thrice weekly moderate load and moderate intensity resistance exercises compared to usual care (stretching exercises) in 27 people with ALS. After three months, when the results of the two trials were combined (43 participants), there was a significant mean improvement in the Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS) measure of function in favour of the exercise groups (mean difference 3.21, 95% confidence interval 0.46 to 5.96). No statistically significant differences in quality of life, fatigue or muscle strength were found. In both trials adverse effects, investigators reported no adverse effects such as increased muscle cramping, muscle soreness or fatigue Authors' conclusions: The included studies were too small to determine to what extent strengthening exercises for people with ALS are beneficial, or whether exercise is harmful. There is a complete lack of randomised or quasi-randomised clinical trials examining aerobic exercise in this population. More research is needed.