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Having the tremendous industrial importance of thermoplastics and elastomers in mind, it is not surprising to see a proliferation of studies on a variety of catalytic systems for polymerization and oligomerization of unsaturated hydrocarbons. Over the last 15 years, the development of mid- to late transition metal catalysts has provided significant advances in this area. The availability of iron combined with its low environmental impact and its tolerance to heteroatom functions attracts significant interest from both academia and industry. In the late 1990s, key milestones have been the development of well-characterized bulky bis(imino)pyridine-Fe(II) precatalysts, mainly for the polymerization or oligomerization of ethylene. This chapter provides a brief overview of the key developments reported in the last 5 years in the literature in the field of iron-catalyzed olefin and diolefin polymerization and oligomerization. Emphasis has been placed on ethylene oligomerization and polymerization, with a particular interest in ligand architecture modifications. The advances in characterization and understanding of catalytically active iron species and the corresponding mechanisms are reported. Heterogenization of bis(imino)pyridine iron catalytic systems has been considered for ethylene transformation and will also be covered in this chapter. The interest of iron catalysts for multiple single-site approaches such as reactor blending and tandem catalysis is also described. Finally, iron catalyst systems also present interesting features for diene polymerization even though both activities and selectivities remain far from those observed for conventional catalysts.