Research has highlighted that adolescent girls are insufficiently active which has serious implications for their current and future health. Walking is recognised as an effective way of implementing regular, health enhancing physical activity (PA) into the daily routine of the general population and in an adolescent population walking is a convenient alternative to active play and sports participation. However it is currently difficult to promote walking and walking initiatives with adolescent girls due to lack of well-established evidence regarding both the quantity and quality of walking that might be advocated to promote PA and health. Therefore the aim of this thesis was to provide step based guidelines with regard to both the quantity and quality of walking required for health and thus inform walking interventions in adolescent girls. In order to achieve this aim four studies were undertaken. Study one explored the most appropriate way to assess walking activity, specifically whether walking on a treadmill accurately replicates walking overground in adolescent girls. Treadmill walking was found to overestimate the metabolic cost of walking in this population. This indicated that studies with the aim of exploring or promoting moderate intensity walking should focus on overground walking. Study 2 explored the quality of walking (steps·min-¹) required to achieve moderate intensity physical activity (MPA) overground. The influence of different anthropometric measures on step rate (steps·min-¹) equating to MVPA were also compared. Results suggest that a generic step rate of 120 steps·min-¹ and 7200 steps in 60 minutes may be advocated to achieve MPA in adolescent girls. However inter-individual variation in step rate associated intensity was observed and it was suggested that a step rate range based on the girl’s body mass may be beneficial for use with adolescent girls. Study 3 considered the most appropriate step measurement instrument to assess free-living walking. Five commercially available instruments (activPAL™ and pedometers; Omron HJ-720-ITC, Omron HJ-304-E, New Lifestyles NL-1000, Yamax CW-701) were compared to direct observation hand-tally step counts, during continuous (study 3a) and incidental (study 3b) walking overground. The New lifestyles NL-1000 was most consistently accurate in quantifying steps and ‘activity time’ during continuous walking, but not during incidental walking. However due to the ease of use and additional youth friendly design features, the New Lifestyles NL- 1000 was utilised in study 4. Study 4 explored the quantity of walking (steps·day-¹) required for health in adolescent girls. The results indicated that in terms of walking activity, ‘healthy’ adolescent girls do not walk significantly more in term of steps∙day⁻¹ or time spent in activity than girls classified as at ‘health risk’. Therefore specific thresholds for quantity and quality of walking required for health could not be defined for this population. Overall findings of this thesis highlight, that walking should be assessed overground with an appropriate measurement instrument. A step rate of 120 steps·min-¹ and 7200 steps in 60 minutes may be advocated to achieve MPA in adolescent girls. However further research is required to explore the relationship between walking and health in this population before we can promote an appropriate threshold of walking that is conducive to good health in adolescent girls.