Biosensors sit at an extraordinary meeting point. Biology supplies recognition, chemistry supplies reactions and surfaces, physics supplies measurable change, engineering builds reliable systems, mathematics quantifies performance, and computing turns signals into decisions. This book introduces that meeting point without hiding its complexity. Every chapter moves from a friendly situation to scientific explanation, a visual model, a practical investigation, critical thinking, and a design mission. The goal is not simply to know sensor vocabulary. It is to reason like a biosensor scientist: define the question, control the sample, understand the signal, quantify uncertainty, test failure modes, and communicate only what the evidence supports.