Do t learn the tricks of the trade, learn the trade I started teachinggraduate coursesin chemical sensors in early 1980s, ?rst as a o- quarter (30 h) class then as a semester course and also as several intensive, 4-5-day courses. Later I organized my lecture tes into the ?rst edition of this book, which was published by Plenum in 1989 under the title Principles of Chemical Sensors. I started working on the second edition in 2006. The new edition of Principles of Chemical Sensors is a teaching book, t a textbook. Let me explain the difference. Textbooks usually cover some more or less narrow subject in maximum depth. Such an approach is t possible here. The subject of chemical sensors is much too broad, spanning many aspects of physical and analytical chemistry, biochemistry, materials science, solid-state physics, optics, device fabrication, electrical engine- ing, statistical analysis, and so on. The challengefor me has been to present uniform logical coverage of such a large area. In spite of its relatively shallow depth, it is intended as a graduate course. At its present state the amount of material is more thancan be coveredin a one-semestercourse (45h). Two one-quartercourseswould be more appropriate. Because of the breadth of the material, the sensor course has a somewhat unexpected but, it is hoped, bene?cial effect.