This book is organized as follows. In Chapter 1 a brief summary of concrete applications involving the sensor location problem is given. Some of these examples are used throughout the monograph to motivate and illustrate the demonstrated developments. A concise general review of the existing literature and a classication of methods for optimal sensor location are presented ; Chapter 2 provides a detailed exposition of the measurement problem to be discussed in the remainder of the book and expounds the main complications which make this problem difficult. In Chapter 3 our main results for stationary sensors are stated and proved. Their extensions to the case of moving internal observations are reported in Chapter 4. ; A more realistic situation with non-negligible dynamics of the vehicles conveying the sensors and various restrictions imposed on their motions is also studied therein and the whole problem is then formulated as a state-constrained optimal-control problem. ; Chapter 5 establishes some methods to overcome the difficulties related to the dependence of the optimal solutions on the parameters to be identied. Finally some concluding remarks are made in Chapter 6.