An important goal of geostatistical modelling is to assess output uncertainty after processing realizations through a transfer function; in particular, to assess the uncertainty in the reserves. The decisions regarding stationarity and a modelling method are critical for obtaining reasonable results. Uncertainty in reserves is affected by the amount of local data and uncertainty in the modelling parameters. Oftentimes, the uncertainty in the input parameters, such as mean, univariate distribution and variogram, to the geostatistical model is ignored. As result, global uncertainty is underestimated. The understatement of uncertainty is especially significant for large reservoirs with sparse well control/ local fluctuations above and below average 'cancel out' and the realizations imply a very small uncertainty. Accounting for uncertainty in the parameters, especially the mean, is very important for a realistic assessment of uncertainty. The objective of this paper is to review methodologies developed for quantification of parameter uncertainty and describe guidelines for incorporation of parameter uncertainty in the reserves calculation. The importance of parameter uncertainty in the assessment of the reserves is also documented.