The term reserve growth refers to the increase in the estimates of ultimately recoverable resources (URR) of known fields over time. Reserve growth has contributed significantly more to reserve additions than new discoveries over the past decade and is expected to continue to do so in the future. But despite the crucial importance of reserve growth for future global oil supply, it remains both controversial and poorly understood. There is a great deal of work to be done before reliable estimates of future reserve growth can be made. This entails both the collation of adequate and reliable fieldlevel data from which to extrapolate future reserve growth, and updating and refining the very rough and preliminary forecast made by the US Geological Survey (USGS), which remains the most comprehensive study to date. "Unpacking" the definition of reserve growth down to its constituent elements reveals that there are a number of definitional issues still to be resolved, in particular regarding the definition of reserves themselves and what categories of oil should be included. For the purposes of estimating reserve growth, it must be clearly defined what categories of oil are considered as their growth characteristics may be different.