 This study will enroll people with heart failure who are participating in a 12-week cardiac rehabilitation program and people with heart failure who are not participating in a cardiac rehabilitation program. All participants will attend study visits at baseline, Week 12, and Month 12. At each study visit, participants will undergo neuropsychological testing, heart rate and blood pressure measurements, and a walking exercise test. They will also complete questionnaires to assess diet, physical activity, and stress levels. Participants will undergo an imaging procedure to measure blood flow in the neck and head, and some participants will also undergo a magnetic resonance imaging MRI procedure of their brain. For 1 week after each study visit, participants will wear a physical activity monitor. At Months 6 and 9, participants will complete questionnaires and mail these back to study researchers. 