Catalysis based on well-defined iron complexes has experienced a very strong growth over the last decade, driven by the need to develop a chemistry that is increasingly in line with environmental concerns, such as the optimization of the planet's resources and the management of the waste from any industrial transformation. The abundant and non-toxic dual character of the iron element is fully consistent with this approach. After the pioneering work of Gibson and Brookhart related to the polymerization of ethylene, which will be briefly recalled here, it appeared in the literature an increasing number of studies on the coordination-insertion polymerization of a large variety of organic monomers using iron-based catalytic systems. The purpose of this review is to provide an update in this field by examining the catalytic systems developed in recent years. Particular emphasis is placed on those involving discrete complexes through their structure and catalytic performance, in terms of both activity and selectivity (when relevant) as well as their limitations. Some key ideas on the salient aspects of this chemistry conclude the manuscript and prospective avenues are put forward.