Background: People with intellectual disabilities (ID) undertake extremely low levels of physical activity. Aims: To enhance understanding concerning low levels of physical activity in people with ID, this study has three aims: (1) to identify barriers to and facilitators of physical activity in people with ID; (2) to examine differences in barriers and facilitators between levels of ID (mild, moderate, severe, and profound); (3) to examine differences in barriers and facilitators between various stakeholder groups. Methods and procedures: A systematic search was performed using the following databases from the year 1990: MEDLINE, ERIC, and PsycINFO. The studies included were peer reviewed, available as full text, and written in English, addressing barriers to and facilitators of physical activity in people with ID. The quality of the studies was assessed using existing critical review forms. All studies were subjected to qualitative synthesis to identify and compare barriers and facilitators. Outcomes and results: In all, 24 studies were retrieved, describing 14 personal and 23 environmental barriers and/or facilitators. The quality of the studies varied, particularly for qualitative studies. Only two studies included people with severe or profound ID. Stakeholder groups tend to identify barriers outside their own responsibility. Conclusions and implications: Results reveal a broad range of barriers and facilitators, but not for people with more severe ID. Further research should also examine these factors among stakeholders responsible for providing support.