The effectiveness of physical activity (PA) for reducing the morbidity and mortality in individuals with chronic disease (CD) has been widely established. CD populations participate in lower levels of PA than their healthy counterparts. Currently research has focused on individual disease cohorts and limited evidence exists investigating levels of PA and SB in a mixed CD population. Study I used a cross-sectional study design to evaluate total daily PA and SB in men and women (62.98 ± 10.99 yr and 50.6% men) with a variety of CDs and examined the association between these behaviours and selected health indices. Participants spent 9.5 h.day-1, 4.1 h.day-1, 1.4 h.day-1, and 0.3 h.day-1 sedentary, standing, in light intensity PA (LIPA) and moderate to vigorous intensity PA (MVPA), respectively and on average took 6713 steps per day. The majority of SB was accumulated in bouts lasting ≥ 30 min. A higher daily step count was associated with more favourable measures of body composition, aerobic fitness and self-rated health. Increased daily sedentary time was associated with less favourable lower body strength. These findings highlighted that individuals with CD, regardless of specific condition, are a target cohort for intervention. Community-based exercise rehabilitation (CBER) programs have the potential to improve health outcomes in CD groups by increasing PA levels and reducing sedentary behaviour (SB). Historically such programs have involved single CDs. Given the similar programme design and the growing prevalence of multimorbidity (MM), an integrated model may be more suitable. MedEx Wellness is a novel CBER service in Ireland, which offers a shared programme to a range of CDs. In study II a quasi-experimental design was used to investigate the effects of a CBER program on levels of PA, SB and selected health indices, in men and women with a variety of CDs. There were significant improvements in LIPA, patterns of SB, physical function, body composition and psychological health following participation in a CBER program. A higher attendance to the CBER program was associated with improvements in measures of LIPA, MVPA, step count, time in sedentary bouts > 60 min, physical function, body composition and psychological health and psychosocial determinants of PA. These findings demonstrate that CBER is an effective approach to increasing PA and improving health related outcomes for individuals with CD, however statistically significant findings often mask the range of inter-individual variability that exists within response to CBER. In study III factors associated with an effective response to participation in a CBER, in terms of a measurable change, in men and women with a variety of CDs were explored. For measures of LIPA, strength, body composition and psychological health, a lower baseline (BL) value increased the likelihood of achieving a measurable change. A higher cardiorespiratory fitness (CRF) level at BL, increased the likelihood of achieving a measurable change. Individuals with cancer were more likely to improve measures lower body strength. Individuals with metabolic disease and respiratory disease were less likely to improve measures of LIPA and psychological health, respectively. Individuals with CD participate in low levels of PA and accumulate high levels of SB. A shared CBER program is an effective approach to inducing change in PA, SB, physical function and psychological health in a CD cohort. Identifying factors associated with a non-response to CBER could optimise program design and delivery.