Sedentary behaviour involves low energy activities undertaken whilst sitting or lying down. Engagement in high levels of sedentary behaviour is a risk factor for obesity, diabetes, heart disease, and premature death, and these risks are largely independent of how much a person engages in moderate or vigorous physical activity. Older adults represent the age group in which sedentary behaviour is most common, therefore reducing sedentary behaviour is a particularly promising target for this population. This PhD aimed to investigate the acceptability of reducing sedentary behaviour for older adults, and to use this knowledge to develop an intervention in line with best practice. A further methodological objective involved investigating the how the Theoretical Domains Framework (TDF) has been used in qualitative research involving patient or public populations. A qualitative interview study (Study 1) was conducted with older adults from diverse socioeconomic areas, using the TDF to inform data collection. This study resulted in two papers, one relating to older adults' construal of sedentary behaviour and one identifying factors associated with older adults' engagement in sedentary behaviours and barriers and facilitators to reducing their sedentary behaviour. We used the TDF in Study 1 as it can facilitate comprehensive theoretical assessment of the determinants of a given behaviour. However, prescriptive use of the TDF may risk qualitative research becoming overly theory-driven, to the detriment of novel inductive insights. We conducted a rapid systematic review investigating specific ways the TDF has been used in data collection and analysis of qualitative studies and the impact of such methods on study findings (Study 2). The findings from Study 1 informed the development of an intervention to reduce sedentary behaviour in older adults, nested within a community physical activity programme for older adults in a deprived area of Manchester. A mixed methods evaluation of the feasibility and acceptability of the intervention, and feasibility of intervention methods and evaluation is provided (Study 3). Older adults appear to conflate reducing sedentary behaviour with increasing physical activity, and previous work has found increasing physical activity to be unacceptable to older adults. An intervention described as a sedentary behaviour reduction intervention may therefore be unappealing to many older adults. An education element within interventions may be useful in challenging the notion that reducing sedentary behaviour necessitates increasing moderate-vigorous physical activity. Providing examples of ways in which sedentary behaviour may be reduced, without involving moderate-vigorous physical activity, may also prove effective. Additionally, education could be provided on the independent effects of sedentary behaviour on health. Further, older adults desire social interaction, therefore community interventions should try to reduce sedentary behaviour by offering group-based, social activities, particularly in areas of high deprivation where current provision is limited. The TDF is often applied prescriptively within qualitative research involving behaviour change of patient or public populations. We suggest that qualitative work could use the TDF in conjunction with a less prescriptive and more inductive approach to research to ensure that the context and detail gained from an inductive approach to data collection and analysis is not lost. We have developed a sedentary behaviour change intervention based on information provision delivered within a community group setting, which has shown evidence of feasibility and acceptability to older adults. These studies provide a foundation for a future definitive trial to reduce sedentary behaviour in older adult populations.