As an oil field matures, the need for reservoir information increases. Improved geological descriptions are frequently necessary for the implementation of numerical modelling or improved oil recovery techniques. Frequently this description can be enhanced by tracer studies. Detecting the location of faults between wells is one specific use for tracing fluid flow. Tracer studies have also been used to complement the information provided by other reservoir characterization methods such as pulse and interference testing. The primary importance of tracer testing is that it provides direct proof that fluid flow has progressed from point A to point B, showing that communication paths exist. In addition, measured tracer breakthrough times are quite useful for inferring the flow characteristics of the reservoir. If the tracer program is poorly designed, the apparent absence of tracer detection at point B may lead to incorrect decisions affecting the management of the reservoir. We will discuss the proper selection and usage of chemicals for tracer testing. Limitations and precautions for individual chemicals will be demonstrated with test data from several field tests. The objective of a tracer test design must be known before a proper test can be implemented. Environmental aspects and analysis of certain tracer materials will also be addressed.