Abstract: A hydroamination reaction is the addition of the N-H bond of an amine across the π-bond of an unsaturated hydrocarbon. This transformation is particularly efficient in comparison to other methods for the synthesis of substituted amines because it directly couples the alkene and amine reagents in a single step instead of proceeding through multi-step pathways and oxygen-containing intermediates. Multiple transition metal complexes have been shown to catalyze the additions of N-H bonds to alkenes and alkynes; however, the substrate scope of these reactions is usually narrow and a general method for the hydroamination of unactivated alkenes has not yet been realized. The first two chapters of this dissertation describe the development of a new tantalum-catalyzed alkyne hydroamination method and an investigation of the stoichiometric reactivity of the tantalum-imido catalysts with unsaturated substrates. The last chapter describes the discovery of a Brønsted-acid catalyzed hydroamination method for styrenes and cyclic alkenes. Chapter 1. The tantalum imido complexes Bn₃Ta=NCMe ₃ (1a), Np₃Ta=NCMe₃ (1b), [Bn₂Ta=NCMe₃][B(C₆F₅)₄] (2), (Et₂N)₃Ta=NCMe₃, and Ta(NMe₂)₅ have been identified as effective catalysts for the hydroamination of alkynes with anilines. Complexes 1b and 2 have been identified as the most active catalysts for these transformations and cationic tantalum complex 2 has shown enhanced reactivity towards more challenging substrates such as ortho-substituted anilines and allenes. Interestingly, cation 2 is also one of the first two early metal complexes shown to catalyze the intermolecular hydroamination of norbornene with aniline. A competing addition of an ortho-C-H bond of the aniline arene ring across the alkene π-bond has been observed with this transformation. A catalyst decomposition product has also been identified as an octahedral tantalum imido cluster with a central heteroatom. The tantalum cluster can be synthesized independently from 1a and aniline. Chapter 2. The synthesis and reactivity of neutral and cationic tantalum imido complexes are described. (Abstract shortened by UMI.)