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. Objectives: The objective was to systematically review randomised and quasi-randomised studies of exercise for people with ALS or MND. Search strategy: We searched the Cochrane Neuromuscular Disease Group Trials Register, EMBASE (January 1980 to August 2007), LILACS (January 1982 to August 2007), MEDLINE (January 1966 to August 2007), Cochrane Central Register of Controlled Trials (CENTRAL), PEDro (January 1980 to August 2007), AMED (January 1985 to August 2007), HealthSTAR (January 1975 to August 2007), CINAHL (January 1982 to August 2007). We also searched Dissertation Abstracts, inspected the reference lists of all papers selected for review and contacted the 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 amyotrophic lateral sclerosis, 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. Data collection and analysis: Two review authors independently assessed trial quality and extracted the data. The authors of the papers were contacted to obtain information not available in the published articles. Main results: We identified two randomised controlled trials that met our inclusion criteria. The first examined the effects of a twice-daily exercise program of moderate load, endurance exercise versus "usual activities" in 25 people with ALS. The second 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, there was a significant weighted mean improvement in the Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS) measure of function in the exercise compared with the control groups (3.21, 95% confidence interval 0.46 to 5.96) in favour of the exercise group. No statistically significant differences in quality of life, fatigue or muscle strength were found. Authors' conclusions: The only studies detected 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.