Introduction The historical background of water flooding is well known to most engineers associated with the petroleum industry. The earliest water floods were accidental in nature resulting from improperly plugged wells or casing leaks. The obvious benefits of these accidental floods soon encouraged the operators to inject water intentionally. The early waterflooding techniques were thoroughly developed and standardized long before even the most elementary concepts of reservoir mechanics were understood. Under these circumstances, the evaluation of waterflood prospects was strictly a matter of analogy and application of "rules of thumb." If a successful water flood had been conducted in a certain sand in a certain area, operators would presume that a nearby field producing from the same zone could be flooded with similar results. Use of analogy and rules of thumb had obvious limitations, as is apparent from the rather frequent unsuccessful waterflood attempts. Reliance upon such methods is still surprisingly heavy even today. Modern reservoir engineering enables us to determine the range of reservoir conditions which will result in favorable response to waterflood efforts, and also to delineate the range of conditions under which water flooding will be of doubtful success. By means of relating the various reservoir parameters involved to the results obtained from water floods, a more reliable set of experience factors can be obtained and the uncertainties involved in water flooding greatly reduced. As is illustrated herein, there are a number of reservoir factors which have a profound influence upon the success of a waterflood project and unfavorable values for any one or two of these factors can result in complete failure of a flood, even though other factors may be quite favorable. It is hoped that this discussion of the effect of various reservoir parameters upon waterflood results will, if nothing else, point out the severe limitations of the rules of thumb by which so many decisions to water flood have been made. In order to illustrate the manner in which the various reservoir factors operate and their relative importance, it has been necessary to assume a certain set of hypothetical reservoir conditions and investigate the effect of varying one or two of the factors independently. It must be emphasized that the values for waterflood recovery efficiency derived herein are of no real significance except for those specific conditions assumed and must not be used directly for the purpose of evaluating any given waterflood prospect. The term "waterflood recovery" as used herein refers to the increase in recovery over and above that which would be obtained by producing a field to its economic limit by primary means. In the case of a flood initiated before all of the primary oil has been produced, the recovery from the date of inception of a flood is the sum of the remaining primary recovery plus the "waterflood recovery" as defined herein.