Catalytic hydrogenation stands out as a powerful tool for the atom-economical transformation of unsaturated systems. Nitrogen heterocycles are widespread in organic chemistry, they are found in many natural products and exhibit notable biological activities. Among them, piperidine is the most commonly encountered saturated heterocycle in alkaloids and drug discovery. Its high sp3 carbon-content contributes to higher drug-success rate and enables access to broader chemical space, escaping from traditionally flat sp2-rich drug molecules. Piperidines are notoriously difficult to access scaffolds and there does not seem to be a single strategy that would enable their streamlined synthesis. In contrast, pyridines are very easy to access and represent a highly modulable core, thus hydrogenation provides a simple way to access complex piperidines from readily available pyridines. However, the hydrogenation of pyridines remains underexplored compared to other heterocycles and most reports rely on a three-step strategy involving activation, hydrogenation and deprotection to access piperidines. Our first objective focused on the development of a catalytic system that would enable pyridine hydrogenation without the need for prefunctionalization. We found that, using an air and moisture stable C,N-cyclometalated iridium complex with a strong acid enabled the hydrogenation of a broad scope of (highly) substituted pyridines. Our method tolerates a large range of functional groups delivering unique and sometimes unprecedented functional group combinations on the piperidine ring, allowing fast access to new chemical space. We then imagined a new design of chiral catalysts that showed good promises in the enantioselective hydrogenation of pyridines. A library of chiral iridium complexes was synthesized and tested to assess what substitution could improve the catalyst performance. Further ligand design will need to be investigated in order to improve the catalyst efficiency and enantio-induction. The hydrogenation of complex nitrogen-containing-heterocycles was then investigated using the previously established methodology. This allowed us to access many piperazines under mild conditions and to explore the hydrogenation of pyridazines to piperidazines, without hydrogenation of the sensitive N-N bond. Several bicyclic systems were also readily hydrogenated, giving access to tetrahydronaphthyridines, tetrahydroazaindoles or tetrahydropyrrolo[1,2-b]pyridazines.