Physical activity (PA) has potential benefits after stroke or spinal cord injury (SCI), especially in improving efficiency and functional capacity in activities of daily living. Currently, many who could benefit from PA may be routinely excluded from participation because of myths related to functional capacity and the concern for harm. The purpose of this review was to evaluate the literature for reports of adverse events during exercise after stroke or SCI, and to provide recommendations regarding exercise participation in supervised and unsupervised environments. Studies were evaluated for quality, and the summary level and quality of evidence were evaluated using the AGREE rubric, modified to address the main outcome measure of adverse events. Levels of exercise stress were evaluated for aerobic activities, using an established rubric. Included in the current analysis were 32 studies for stroke and 4 for SCI. In aggregate, this yielded a total of 730 experimental participants with stroke and 143 with SCI. It should be noted that almost all studies were not designed to examine naturally occurring adverse events from PA. Significant contraindications to unsupervised exercise include manifestation of autonomic dysreflexia in SCI and cardiovascular comorbidity after stroke. There are clear benefits of exercise training on physiological outcomes in stroke and SCI, but the relation between outcomes and safety remains unclear. However, taken on balance, the risk-to-benefit ratio favors the recommendation of exercise. This recommendation is based on studies in which participants were almost universally screened for participation in supervised environments. Thus, the grading of evidence for finding adverse events to support this conclusion is inadequate.