This textbook presents a comprehensive theoretical analysis of the physics underlying semiconductor materials used in gas sensing applications.  Beginning with the fundamentals of quantum mechanics and solid-state physics, the text systematically explores the physical principles and operational mechanisms of chemoresistive sensors. Through an innovative approach and a depth of coverage not found in existing literature, it offers an advanced and up-to-date treatment of sensor physics. The content is primarily designed for students enrolled in the ?Sensors: Physics and Technology? course within the Master?s Degree Program in Physics at the University of Ferrara (UNIFE). However, it also serves a broader academic audience, including Ph.D. candidates, research fellows, and researchers in solid-state physics, particularly those specializing in gas sensing and chemoresistive sensors based on semiconductor materials.