How to effectively and efficiently reduce carbon dioxide (CO2) to value-added chemicals represent a frontier in catalysis research. Due to the high activation energy needs and the endothermic nature of CO2 reduction, the reactions are difficult to carry out. When H2O is present, hydrogen evolution reactions (HER) often compete favorably with CO2 reduction reactions. For these reactions, catalysts are of critical importance to CO2 reduction. In this article, we review the various metal nanocatalysts for electrochemical CO2 reduction (ECR) reactions. In recognition of the importance of H2O to CO2 reduction, we focus our discussions on systems in aqueous solutions. Nanostructured metal catalysts are chosen for the discussions because they represent the most effective catalysts for ECR. After a brief introduction of the fundamental principles of ECR, we devote the rest of the article on the discussions of various types of nanostructured metallic catalysts, which are categorized by their compositions and working mechanisms. Lastly, strategies for improving reaction efficiency and selectivity are discussed. (C) 2018 Elsevier Inc. All rights reserved.