The ALSPAC dataset can answer the research questions we have efficiently and quickly, and with many major scientific strengths. It is the largest sample of its kind which is longitudinal, has good measures of the relevant exposures (total volume of habitual physical activity and intensity of physical activity measured by accelerometry at age 11 and 13y), of confounders, and of outcome measures (cognitive measures, academic attainment, mental health and well-being as measured by the SMFQ and the SDQ). Almost all previous studies to address our research questions have suffered from a combination of small sample size, cross-sectional design, crude or biased measures of exposures and/or outcomes, limited consideration of confounders.Please note that we already have the data required, as part of an existing collaboration with ALSPAC on a Bupa Foundation funded secondary analysis (ALSPAC coapplicants Prof Ness, Dr Leary, Dr Joinson; data buddy Dr Northstone) of associations between physical activity and cognition, academic attainment, and mental health in typically developing 11-13y olds in ALSPAC. The current proposal adddresses essentially the same research questions, but in study participants excluded from our current study on the grounds that they have developmental disorders.