Childhood obesity and its repercussions will be one of the most serious public health challenges of the 21st century. Promoting physical activity and preventing the decline of children's fitness and the increase of childhood obesity, diabetes, and cardiovascular disease therefore need to be a high priority. So far, besides a few large-scale studies conducted in developed countries such as the United States, Australia, and European countries, few have been conducted or reported in other countries, including China, a country with 20% of the world's population. Although there has been a well-developed youth physical fitness surveillance system in China, no major national study has been conducted there to measure children's physical activity until recently. Sponsored by the Chinese National Science & Technology Pillar Program, a major national physical activity survey study called Chinese City Children and Youth Physical Activity Study (CCCYPAS), which the author of this article directed, was completed recently. Specifically, between April 2011 and May 2012, physical activities of a total of 3,121 healthy children and youth (1,603 boys and 1,518 girls; Grades 4-11) from 11 cities in China were surveyed using ActiGraph accelerometers. The participating cities include Chengdu, Fuyang, Ganzhou, Guangzhou, Shanghai, Shenyang, Tianjin, Tongzhou, Wenzhou, Xian, and Yingtan, which are spread across China. Major steps of the CCCYPAS included: (1) Forming and training the research team; (2) Getting the human research protocol approved; (3) Recruiting participants; (4) Data collection; and (5) Data analysis and preparing reports. In addition to the key findings of the CCCYPAS, such as daily moderate-to-vigorous physical activity (MVPA) minutes and number of daily steps taken by Chinese city children and youth, several measurement and analytical studies were completed simultaneously using the CCCY-PAS sample or data. Publishing the findings of these studies in English will not only inform the world for the first time of the physical activity status of Chinese city children and youth, but it will also hopefully make a meaningful contribution to the literature regarding physical activity participation of children and youth around the world and its related issues. A total of nine articles are included in this supplement. The first two studies focused on determining the number of daily MVPA minutes in which children and youth engaged through the use of accelerometers. The third study confirmed the low physical activity participation that had been observed in prior studies. The fourth and fifth studies focused on missing data in physical activity research. The sixth through eighth studies joined the effort to explore solutions. Finally, in the ninth study, Wang, Chen, and Zhuang examined the validity and reliability of the International Physical Activity Questionnaire-Short Form (IPAQ-SF) using the CCCYPAS data.