To investigate how people determine the intensity of their physical activity, respond to instruction, and their understanding of moderate intensity walking.Experimental.Forty nine regular walkers had their walking speed, heart rate and rate of perceived exertion measured during their normal walking behaviour, when instructed to walk at a moderate intensity, and after provision of feedback on how to meet moderate intensity walking.Although moderate intensity walking was generally perceived as too low to be beneficial, walking speed was the best variable matched across all experiments for moderate intensity (83.7%, 67.3% and 93.9%), followed by heart rate, with a large proportion of participants responding favourably to instruction relating to heart rate and speed to meet moderate intensity walking. Rate of perceived exertion was a poor reflector of walking intensity.Most participants naturally selected a walking intensity that elicits moderate intensity and when instructed to walk at moderate intensity, the intensity is more vigorous. Attempting to match speed and heart rate limits for moderate intensity reduced the intensity perceived by participants.