Abstract The recent publication for discussion of the SPE/WPC draft reserve definitions1 has highlighted the difficulty in attempting to relate deterministically-derived reserve estimates with those based on a probabilistic methodology. Although the debate over the benefits/disadvantages of the two estimation approaches will no doubt be rekindled, a more fundamental issue is that of the difference between an "incremental" (or risk) based philosophy to reserve estimation and a "Best Estimate" (or uncertainty) based one. The current SPE definitions2 are based on an incremental philosophy where Proved reserves are "reasonably certain", Probable reserves are "less certain" than Proved reserves and the summation of Unproved (Probable and Possible) reserves with Proved reserves is precluded "because of different levels of uncertainty". The use of "uncertainty" in this context really reflects risk. This philosophy is perfectly acceptable and has been the fundamental basis of reserve estimation (especially in the USA) for many years; however, it is not directly comparable to the philosophy on which the probabilistic approach is based. The Best Estimate philosophy reflects the hypothesis that the primary objectives of reserves estimation are (a) to make the best estimate possible as to what hydrocarbon volumes will actually be recovered up to field abandonment, and (b) to assess what is the uncertainty in that estimate. This philosophy is inherent in the probabilistic approach, but it is clear that the deterministic methodology can also be (and is) applied using this logic. If the Best Estimate philosophy is applied, deterministic and probabilistic reserve estimates may be comparable (in theory at least).