In this chapter, recent developments of so called non-metallocene early transition metal (group 3-5) complexes as the catalyst precursors for olefin polymerization have been described. These catalysts display unique characterisitics especially for ethylene (co)polymerization, and high isospecificity ([mmmm] [ 99.6 %) in propylene polymerization has been achieved in some catalyses. The ligand modification plays an essential key role not only for the catalytic activity, but also for control of tacticity and/or living nature in these catalyses.