This paper discusses the use of pressure transient tests to obtain a quantitative reservoir description. Interpretation of test data (using both analytical and numerical models) is described, and the advantages and limitations of such models are pointed out. Tests that should be applied to the data to determine the applicability of different models also are described. Field data and their analyses are presented. Introduction Enhanced oil-recovery processes require a relatively accurate reservoir description. In these processes, a chemical slug or a thermal front is moved between the injectors and the producers. Reservoir description has a significant effect on the design, operation, and economic success of these processes. Since these methods can be applied to the reservoir only once, the need for a reliable reservoir description is obvious. Cores and well logs provide information about the rock permeability and provide information about the rock permeability and porosity and the fluid saturations. The limitations of porosity and the fluid saturations. The limitations of these methods are well-known. They sample a small volume of the reservoir, and the results are affected by the drilling, completion, and testing techniques. Matching the secondary-recovery performance (e.g., production rates, reservoir pressure, WOR, and GOR) of the production rates, reservoir pressure, WOR, and GOR) of the reservoir yields an acceptable description and should be used as a starting point when determining the feasibility of enhanced recovery methods. Indications are, however, a better description is needed because the relatively wide spacing commonly used during secondary recovery does not produce the detailed description of smaller spacing, which produce the detailed description of smaller spacing, which frequently is required for enhanced recovery. Tracers and pressure transient tests are two techniques that can be used to describe reservoirs. To date, pressure transient tests have been used more extensively than tracers. Therefore, more types of pressure tests are known, theory is better developed in this area, and experience with pressure tests far exceeds that with tracers. Determining volumetric sweep efficiency is a problem that tracers potentially can solve best. Currently, problem that tracers potentially can solve best. Currently, describing reservoir heterogeneity by matching tracer performance is hampered by (1) lack of adequate performance is hampered bylack of adequate numerical models,the long times that are required to obtain results, andthe dependence of the match on additional parameters that are introduced by the tracers themselves (e.g., dispersion coefficients, tracer retention). Promising new solution techniques' are being developed in this area, however, and it is quite feasible that tracers and pressure transient tests will be used together in the future for reservoir description. Several types of pressure transient tests are available. This includes both single- and multiple-well tests. The theory of fluid flow in a homogeneous medium (either isotropic or anisotropic) is well-developed. Analytical solutions exist for analyzing pressure transient data. For heter ogeneous reservoirs, no set guidelines exist for analyzing tests so reservoir description can be obtained. Here, a systematic method is proposed for analyzing pressure transient tests for reservoir description. The pressure transient tests for reservoir description. The properties that need to be determined are porosity, (x, properties that need to be determined are porosity, (x, y, z); thickness, h(x, y); and permeability, k(x, y, z). The use of multiple-well tests to determine these properties in different systems is considered. Different models and analysis techniques are discussed and their advantages and limitations pointed out. Tests that should be applied to data to determine the applicability of different models are described. JPT P. 1060