When we stop and think about what is truly important in life, most of us would say that it is quality time spent with family and friends and the ability to do what we enjoy. This is also the primary goal for those with end-stage heart failure (HF) fortunate enough to get a second chance at life through heart transplantation (HTx). Management post-HTx is complex with many factors including lifestyle playing a role. Some people get a little more time, others get decades, but how is this extra time spent? Some HTx recipients have completed events such as an ironman triathlon, which is so mentally and physically gruelling that even the majority of people who haven’t had a HTx would never attempt this. These are exceptional cases, but how physically active are the majority of HTx recipients? We know that in the general population many people are not physically active enough to gain health benefits and that there is a worldwide obesity epidemic, yet little is known about levels of physical activity in the HTx population. Whilst exercise capacity improves post-HTx, it remains unclear how physically active HTx recipients are once they have recovered post-surgically. Just because someone has an improved exercise capacity post-HTx, they may not have an improved level of physical activity. The aims of this thesis are to (1) describe physical activity levels and (2) identify factors which may be associated with levels of physical activity post-HTx. A prospective observational cross-sectional study was conducted at a single centre HTx outpatient clinic. Medically stable adult HTx recipients ≥6 months post-HTx were recruited. Exclusion criteria were left ventricular ejection fraction (LVEF) less than 50% by any imaging modality, a rejection episode within the past two months or any injury or illness that could be reasonably expected to reduce mobility. Physical activity level (PAL) and average daily time spent performing at least moderate intensity activity (≥3 metabolic equivalents (METs)) were estimated using a multi-sensor device (SenseWear Pro3). Factors investigated were demographic (age, sex, body mass index (BMI), time post-HTx and reason for HTx), corticosteroid use, exercise capacity (six minute walk distance (6MWD)) and quadriceps muscle strength corrected for body weight (QS%). Eighty-seven HTx recipients were recruited between February 2013 and August 2014. Of these, 12 were excluded. Therefore, seventy-five participants, (20 female, 26.7%) and time post-HTx 9.2 ± 7.0 years (0.5 to 26 years) were included. Twenty-seven (36%) were classified as extremely inactive (PAL <1.40); 26 (34.6%) sedentary (PAL 1.40 to 1.69) and 22 (29.3%) active (PAL >1.70). Multivariable analysis showed greater QS% (β=0.004 (0.002 to 0.006), p=0.001) to be independently associated with increased PAL. For increased time ≥3 METs, both greater QS% (β=0.0164 (0.003 to 0.029), p=0.014) and lower BMI (β=-0.0626 (-0.115 to -0.0099), p=0.021) were independently associated. These results highlight that quadriceps muscle strength is an important factor with regard to physical activity however causation cannot be stated in this observational study. Additionally, regular participation in physical activity can contribute to weight maintenance, and physical inactivity along with excessive energy intake often results in increased BMI and obesity.