There is substantial uncertainty associated with the inputs characterizing a reservoir, and we force the reservoir models with external controls related to, e.g., injection rates and production constraints. Thus, both conditioning on data for estimating model parameters, and optimization of well-controls for maximizing production or net present value, are necessary. Besides, there is a continuous process of engineering and financial decisions to be made regarding, e.g., placement and drilling of new wells or modifications of production capacity. We address several different problems related to both planning and decision making for field development including optimization of the placements of wells and the drilling order. For producing fields, we will optimize the drilling of infill wells as well as production strategies.