Background: The Take PART study—Physical Activity Research for Teenagers—was undertaken to assess (1) physical activity and sedentary behaviors, (2) indices of health and fitness, and (3) to provide information, from a social ecological perspective, on the correlates of physical activity in a large sample of 15- to 17-year-old Irish adolescents. This manuscript outlines the rationale and methodology of the Take PART study. Methods: A sample of 4720 students (mean age = 16.03 years ± 0.66, range 15 to 17 years; 49.5% female) participated. Fifty participants were assessed during each 3-hour school visit, with a ratio of 1 researcher to 10 students. Standardized testing procedures and extensive researcher training ensured that intertester and intratester reliability for all physical measures was ≥.85. The height, weight, waist circumference, hip circumference, blood pressure, and cardiorespiratory fitness protocols are explained. The questionnaire used well-known, valid, and reliable self-report measures. Where appropriate, additional psychometric testing was undertaken. Conclusions: Take PART is a school-based study. Its methods are simple, easy to replicate, financially viable, and scientifically valid. Its unique dataset will allow the evaluation of a social ecological approach as a viable option for improving understanding of youth inactivity. Ultimately, this knowledge will assist in successful intervention design.