Purpose: The purpose of this study is to capture students' essential knowledge and behaviors concerning active living. Methods: Students (N = 1,079) from elementary, middle, and high schools in the United States reported their knowledge of physical activity and fitness (PAF knowledge), and physical activity and sedentary behavior using grade-specific PE Metrics tests and Youth Activity Profile, respectively. Descriptive and inferential statistical analyses were used to address the research purposes. Results: The total PAF knowledge scores and scores in subareas varied by gender and school level. A declining trend for physical activity and an increasing trend for sedentary behavior were observed. PAF knowledge positively predicted physical activity in elementary school boys and middle school girls and negatively predicted sedentary behavior in middle school students and high school boys. Certain PAF knowledge subareas (e.g., elementary school PD#3: "knowledge about the characteristics of health-enhancing physical activity;" middle school PD#1: knowledge of "physical activity participation as part of a healthful lifestyle;" high school PD#4: "monitoring and adjusting physical activity to meet fitness needs") also significantly predicted behaviors. Conclusion: The findings may guide teachers' curricular and instructional actions to enhance students' PAF knowledge through physical education.