This book's approach to general sensor theory is divided into two parts. Part I deals with the description and composition of sensors and the characterization of the information produced by sensors, whether they be artificial sensors or the sense organs of living beings. The aim here is to define what a sensor is and what methods are used in sensing. The nature of the sensory problem is discussed, the distinction is drawn between sensing and perceiving, and the possible kinds of sensors and sense organs are enumerated. Part II deals with the theoretical and epistemological implications of sensor theory. These include the mature of the sensory object, projections and errors, an information theory model of the sensory process and image analysis. This book is not an engineering manual for the design of artificial sensors, nor is it a discussion of the details of human sense organs or sensation. The aim here is to draw upon these detailed studies and find a theory which encompasses them.