This book contains the 2020-2021 contests organized by the American Computer Science League. ACSL is divided into multiple divisions to appeal to different abilities of students. Each competition in the Senior, Intermediate, and Junior divisions consist of two parts: Theory Problems that cover fundamental concepts in computer science and a Coding Problem that students solve by writing a computer program in a language of their choice. The Elementary and Classroom divisions consist of theory problems only. The 2020-2021 school year was ACSL's 43rd year of continuous operation! About 5,000 students in the United States, Canada, Europe, and Asia participated. We welcome the reader to learn more about the American Computer Science League on its website, www.acsl.org.