Reservoir heterogeneities are known to influence success or failure of infill wells in waterfloods, but no satisfactory ways currently exist for their characterization, quantification; or prediction: In 1950, Dykstra and Parsons((1)) presented empirical cork relation for waterfloods in certain California reservoirs based on observed variations in permeability. In general, procedures for most reservoir engineering predictions could be greatly improved if heterogeneity could be characterized by one or more terms. However; it is painfully obvious that because of the multitude of reservoir descriptions encountered in any oil prone-basin; a single parameter (or a small number of parameters) may not be adequate. This is because of different ways in which heterogeneity impacts the performance of vertical or horizontal infill wells placed to enhance production/reserves in a variety of waterflood situations. It is postulated that reservoirs are divided into predominantly horizontal or vertical "compartments" due to spatial variations in reservoir attributes such as permeability, thickness; environment of deposition, and post-depositional changes. Oil contained in these compartments, though often not strictly isolated, is not easily contacted or displaced by the injected water. Alternately, flow within the reservoir might be predominantly through certain pathways,: some of which might involve cross-flow between various regions or intervals: Horizontal wells do a better job of draining oil than vertical wells in certain situations and vice versa. This paper proposes a method for characterizing reservoir heterogeneity and illustrates its application via four case studies of infill wells in waterfloods in the Western Canadian Sedimentary Basin (WCSB). Comparative performance is presented for infill horizontal and vertical wells placed in these projects during the past 15 years.