Hydrogenation of vegetable oils was first discovered in the beginning of this century and quickly became a major processing step in the fats and oils industry. Hydrogenation changes a liquid oil made of triglycerides of polyunsaturated fatty acids to one containing mostly monounsaturated fatty acids. This procedure improves the functionality and flavor stability of the oil. The process is a complex series of reactions occurring in a three-phase slurry reactor in a batch mode primarily utilizing supported Ni catalysts and can result in a variety of products. A good catalyst for edible oil hydrogenation must have good activity, selectivity, poison resistance and filterability. The physical characteristics of the catalyst affect these attributes greatly. This paper discusses these attributes and how a catalyst and the operating conditions affect them.