Strontium clusters with 2 to 13 atoms have been studied by means of high level quantum chemical ab initio calculations using a scalar-relativistic energy-consistent large-core pseudopotential, a corresponding core-polarization potential, large valence basis sets including up to g functions as well as coupled-cluster and configuration interaction correlation treatments. Equilibrium structures, cohesive energies, vertical as well as adiabatic ionization potentials and electron affinities are reported. Vibrational frequencies for clusters with up to 5 atoms also have been determined. To demonstrate graphically the transition from van der Waals to covalent interactions for increasing cluster size and to comment on the contributions of sp and sri hybridization to chemical bonding, the electron localization function (ELF) has been calculated from the Hartree-Fock wave functions for the equilibrium structures determined in correlated calculations. Strontium clusters behave rather differently from clusters of the group 12 elements as well as ytterbium, i.e., they exhibit significantly larger cohesive energies than ytterbium clusters due to more pronounced covalent contributions to bonding, and much lower ionization potentials and smaller band Saps. These differences can be mainly attributed to the considerable sp as well as sd hybridization in strontium clusters, while only either the former or the latter is important for group 12 and ytterbium clusters, respectively.