This paper describes important reservoir engineering issues of horizontal hole drilling in primary field development, highlighting the areas of productivity Improvement In tight reservoirs and oil rim development. Factors influencing the productivity improvement over that of vertical wells In tight reservoirs are discussed, e.g. horizontal hole length, permeability anisotropy ratio, well location with respect to areal boundaries, hydraulic fracturing and mechanical skin factor. In the area of oil rim development, a discussion on critical rate models (including one which includes pressure losses In the borehole) Is given for gas/oil systems, whilst also showing how the critical rate concept can be used for production forecasting. Applications of numerical simulation are described for more complex geological models for oil rims with both gas cap and active water drive. Among the factors influencing the production performance of horizontal wells in oil rims, oil mobility, drainage area, vertical well position in the oil rim/strength of water drive, well length, permeability anisotropy ratio or the presence of dipping shale layers are described. Well testing is discussed next as a way of obtaining critical parameters such as permeability anisotropy ratio and mechanical skin. Finally a summary of field applications in areas where Shell has an interest is given.