Using a combination of classical molecular dynamics simulation and first principles molecular orbital theory, we provide the first comprehensive study of the equilibrium geometries, energetics, electronic structure, vertical ionization potential, and magnetic properties of Ni clusters containing up to 21 atoms. The molecular dynamics simulation makes use of a tight binding many-body potential, while the calculations based on molecular orbital theory are carried out self-consistently using the numerical atomic bases and the density functional theory. The adequacy of the molecular dynamics results on the energetics and equilibrium geometries is tested by comparing the results with those obtained from the self-consistent molecular orbital theory for clusters of up to six atoms. For larger clusters, equilibrium geometries were obtained from molecular dynamics simulation, and their electronic structure and properties were calculated using molecular orbital theory without further geometry reoptimization. Frozen core and local spin density approximations were used in the molecular orbital calculations. In small clusters (n less than or equal to 6), the calculations were repeated by including all electrons and the gradient correction to the exchange-correlation potential. The calculated vertical ionization potential and magnetic moments of Ni clusters are compared with recent experimental data.