Background. Optimal physical activity levels have health benefits for patients with acute coronary syndrome (ACS) and are an important goal of cardiac rehabilitation (CR). Purpose. The purpose of this study was to systematically review literature regarding short-term effects (<6 months after completion of CR) and long-term effects (>= 6 months after completion) of standard CR on physical activity levels in patients with ACS. Data Sources. PubMed, EMBASE, CINAHL, and PEDro were systematically searched for relevant randomized clinical trials (RCTs) published from 1990 until 2012. Study Selection. Randomized clinical trials investigating CR for patients with ACS reporting physical activity level were reviewed. Data Extraction. Two reviewers independently selected articles, extracted data, and assessed methodological quality. Result were summarized with a best evidence synthesis. Results were categorized as: (1) center-based/home-based CR versus no intervention, (2) comparison of different durations of CR, and (3) comparison of 2 types of CR. Data Synthesis. A total of 26 RCTs were included. Compared with no intervention, there was, at most, conflicting evidence for center-based CR and moderate evidence for home-based CR for short-term effectiveness. Limited evidence and no evidence were found for long-term maintenance for center-based and home-based CR, respectively. When directly compared with center-based CR, moderate evidence showed that home-based CR has better long-term effects. There was no clear evidence that increasing training volume, extending duration of CR, or adding an extra intervention to CR is more effective. Limitations. Because of the variety of CR interventions in the included RCTs and the variety of outcome measures in the included RCTs, pooling of data was not possible. Therefore, a best evidence synthesis was used. Conclusions. It would appear that center-based CR is not sufficient to improve and maintain physical activity habits. Home-based programs might be more successful, but the literature on these programs is limited. More research on finding successful interventions to improve activity habits is needed.