Since publication of the previous, the 3rd edition of this book, the sensor tech- logies have made a remarkable leap ahead. The sensitivity of the sensors became higher, the dimensions – smaller, the selectivity – better, and the prices – lower. What have not changed, are the fundamental principles of the sensor design. They still are governed by the laws of Nature. Arguably one of the greatest geniuses ever lived, Leonardo Da Vinci had his own peculiar way of praying. It went like this, “Oh Lord, thanks for Thou don’t violate Thy own laws. ” It is comforting indeed that the laws of Nature do not change with time, it is just that our appreciation of them becomes re?ned. Thus, this new edition examines the same good old laws of Nature that form the foundation for designs of various sensors. This has not changed much since the previous editions. Yet, the sections that describe practical designs are revised substantially. Recent ideas and developments have been added, while obsolete and less important designs were dropped. This book is about devices commonly called sensors. The invention of a microprocessor has brought highly sophisticated instruments into our everyday life. Numerous computerized appliances, of which microprocessors are integral parts, wash clothes and prepare coffee, play music, guard homes, and control room temperature. Sensors are essential components in any device that uses a digital signal processor.