This article is a review of the area of optimal instrumentation location in process plants for both grassroots designs and upgrades. Emphasis is put in methods for monitoring purposes and based on the cost minimization model. These methods are mostly related to data reconciliation paradigms and include especial consideration to the detection of gross errors. Connections between this model and other models based on other objective functions and goals are analyzed. Finally, the emerging field of sensor design for efficient process fault detection is briefly covered.