This review provides an overview of the literature regarding heterogeneous molecular catalysts for electrochemical CO2 reduction (ECR). Fundamental aspects of the science, including aggregation, electrochemical rate laws, and electrode-catalyst electronic coupling, are discussed to provide a solid foundation on which to design experiments and interpret results. Mechanistic aspects of ECR are presented based on electrokinetic and spectroscopic measurements as well as density functional theory (DFT) calculations. Consensus is improving for electrokinetic measurements, but the redox state of the metal center under reaction conditions and DFT reaction pathways lack agreement in the literature. Concerning the tunable aspects of the molecular catalyst, the impacts of the metal center, ligand substituents, and electrode support on the activity and selectivity toward ECR are presented with an emphasis on those studies that controlled for aggregation and minimized mass-transport limitations. Extended three-dimensional (3D) structures such as polymers, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs) are discussed as highly tunable architectures that begin to mimic the catalytic pockets of enzyme active sites. To achieve the full potential of these catalysts, design principles must emerge based on a combination of deconvoluting measurements to extract intrinsic catalyst properties and more reliable theoretical calculations to predict reaction pathways.