Background: Cheap and effective tools for measuring patients’ physical activity (PA) level are needed. The first aim in this thesis was therefore to assess the validity of two PA -questions, and their three associated answer modes, that are used within the Swedish health care system. Sitting, light intensity PA (LIPA), and moderate and vigorous intensity PA (MVPA), are associated with health and longevity, but detailed population data assessed with objective methods is needed. The second aim was thus to assess the above with motion sensor technology, in a middle-aged Swedish sample. Low self-perceived health is a strong predictor of morbidity and mortality, but this association may vary over time with changes in the society and our lifestyle. The third aim was to assess secular trends in the interrelations between self-perceived health, physical fitness, and selected covariates. The effects of PA on prescription (PAP) on health-related quality of life (HRQoL) in overweight adults are unclear, thus the fourth aim was to explore this. Methods: All data was collected in the Swedish population. Data from the PA -questions and accelerometers, aerobic fitness, counter movement jump, and balance tests, blood samples, and self-rated general health were collected in 365 participants, 21–66 yrs. The PA pattern was assessed in 948 individuals, 50‒64 yrs, from the SCAPIS pilot study. Self-perceived physical health, and measured aerobic fitness, counter movement jump height, and balance, and demographic and lifestyle data, was assessed in three independent samples from 1990, 2000 and 2013, including 3564 adults, 20‒65 yrs. The effects of Swedish PAP on HRQoL was assessed in a randomized controlled trial including 101 men and women, 67‒68 yrs, that were inactive, overweight (BMI>25 kg/m2), and had a waist circumference ≥102 cm (men) or ≥88 cm (women), who were randomized to an intervention group or a control group. The 36-item Short Form Health Survey (SF-36) was used to assess HRQoL. Results: The multiple choice answer mode of the two PA -questions was found to have the strongest validity, compared with the two other (an open mode, and one where PA minutes is specified per weekday). The validity is in line with many other established PA-questionnaires, but the open mode has limitations. The assessment of PA pattern showed that 61% of motion sensor wear time represented sitting, 35% LIPA, and 4% MVPA. Only 7% of the sample met the PA recommendations. The odds for describing perceived health as good was found to increase by 5% per each increment of 1 ml/kg/min in VO2max. This was stable across genders and all three LIV-samples (i.e. over time). Waist circumference, chronic disease, sleep problems, and level of satisfaction with one’s life, were also important correlates. The Swedish PAP group improved significantly more, and more participants displayed clinically relevant improvements (OR 2.43), in mental aspects of HRQoL, compared to the controls. Physical aspects of HRQoL improved in the PAP group, but not in the control group. Conclusions: The multiple choice answer mode has the strongest validity and Open mode the weakest. The PA -questions may be used in populations, or in individuals to determine appropriateness for treatment. The questions’ advantages and limitations must be considered and further reliability and validity studies are needed. The results regarding sitting, LIPA, MVPA and fulfillment of PA recommendations, are of high clinical relevance. A great challenge remains to further implement methods to increase the level of PA in the Swedish population. Physical fitness is related to self-perceived health independently of changes in society and lifestyle over time, and simple questions may be useful for the clinical assessment of physical fitness. Swedish PAP has a positive effect on mental aspects of HRQoL, measured by the SF-36. This finding supports the clinical use of the Swedish PAP model. / LIV 2013