Background: Informal caregivers of patients with amyotrophic lateral sclerosis experience increased levels of caregiver burden as the disease progresses. Insight in the factors related to caregiver burden is needed in order to develop supportive interventions. Aim: To evaluate the evidence on patient and caregiver factors associated with caregiver burden in amyotrophic lateral sclerosis informal caregivers. Design: A systematic review. Data sources: Four electronic databases were searched up to 2017. Studies that investigated quantitative relations between patient or caregiver factors and caregiver burden were included. The overall quality of evidence for factors was assessed using the Grading of Recommendations Assessment, Development and Evaluation approach. Results: A total of 25 articles were included. High quality of evidence was found for the relation between caregiver burden and the factor "behavioral impairments." Moderate quality of evidence was found for the relations between caregiver burden and the factors "feelings of depression" of the caregiver and "physical functioning" of the patient. The remaining rated caregiver factors--"feelings of anxiety," "distress," "social support," "family functioning," and "age"--and patient factors--"bulbar function," "motor function," "respiratory function," "disease duration," "disinhibition," "executive functioning," "cognitive functioning," "feelings of depression," and "age"--showed low to very low quality of evidence for their association with caregiver burden. Conclusion: Higher caregiver burden is associated with greater behavioral and physical impairment of the patient and with more depressive feelings of the caregiver. This knowledge enables the identification of caregivers at risk for caregiver burden and guides the development of interventions to diminish caregiver burden.