Movement is a fundamental behavior that is generated and patterned by premotor neural networks that reside in the ventral spinal cord of vertebrates. Despite ongoing investigations in multiple animal models, including humans, non-human primates, cats and rodents, identifying the functional neuron cell types that shape various motor tasks has proved difficult. One of the key issues in the field is the criteria by which neuronal cell types are characterized and defined. This dissertation describes my work tackling this problem using a comprehensive anatomical approach aimed at identifying neuronal subsets within major classes of ipsilateral premotor spinal interneurons. The first chapter provides a historical background of spinal motor system research developed from the beginning of the 20th century that summarizes our current knowledge of motor circuits in the spinal cord and the contribution that this dissertation makes to bridging the gaps that exist in our understanding of premotor interneuron cell types. The second chapter describes the anatomical approach I pursued, which is based on spatial distribution, connectivity and morphology to identify potential groups of functional premotor interneuron cell types.In the third chapter I discuss the relevance of these findings, by highlighting how the strategy used in this work can be applied to other spinal cord populations and used to lay the groundwork to build an anatomical reference for motor functional studies, including electrophysiology and behavioral assays. This work provides an important and necessary framework to understand the complicated logic governing the organization of motor networks in the vertebrate spinal cord.