| It is well established that physical activity (PA) in mid and later life prolongs life, enhances wellbeing and reduces the risk of many health conditions. Current UK PA guidelines for adults and older adults recommend at least 150 minutes moderately intensive PA weekly, or 75 minutes vigorous PA weekly, both in bursts of at least ten minutes. Objective PA assessment suggest <10% actually achieve recommended levels (much lower than those self-reporting that they achieve them). Increasing PA is, therefore, a key priority for Public Health. There are a range of group and individual based interventions designed to increase PA levels among older people. However this evidence base is problematic because of the low, typically trials recruit between 6 and 35% of those invited, and selective recruitment. Clearly the utility of a trial is hampered if those recruited do not represent those who would be targeted in a ‘real life’ setting In this presentation I will explore the validity of the evidence resultant from PA intervention trials by evaluating the characteristics of those who do (and do not) participate and consider the degree to which hard to reach groups of elders are represented in our evidence base. We then consider why potential participants decline to participate in a PA trial using data collected by the PACE-UP trial which aimed to achieve public health PA targets in those aged 45-75 year old.