Sense and avoid (SAA) represents one of the main roadblocks to the integration of unmanned aircraft systems (UAS) operations by aviation authorities around the world. This tutorial outlines and reviews the substantial breadth of SAA architectures, technologies, and algorithms. Starting from a discussion about what constitutes a UAS and how it is different than manned aircraft, basic SAA definitions and taxonomies are discussed. The SAA process is dissected into three fundamental tasks, defined as sensing, detecting, and avoiding, which are discussed in detail. The tutorial concludes with a summary of the regulatory and technical issues that continue to challenge the progress on SAA, as a key component of reliable UAS operation in civil aviation authorities (CAAs) around the world.