Successful appraisal and development of oil and gas fields requires the integration of uncertain subsurface information into multiple reservoir simulation models. This information includes seismic data, various types of well data, and geologic concepts. Over the past five years, a workflow has been developed by various organizations in conjunction with BHP Billiton Petroleum. This distinctive approach focuses first on building mesoscale reservoir models that can be constrained by seismic data (typically with a resolution up to the stratigraphic seismic loop scale, see Prather et al., 2000), then introducing the finer scale geologic concepts and well data needed for reservoir simulation models (stratigraphic first- and second-order subseismic scale, where each order is about a factor of three in size) via a downscaling step that honors mesoscale model constraints. Uncertainty and correlations of the well and seismic measurements are always taken into account. In fact, they are necessary to be able to combine the various measurements. Bayesian probabilistic techniques are used extensively in this process. The result is an ensemble of reservoir simulation models consistent with all subsurface information.