My talk will examine the relation between visualization and scientific understanding, especially in modern physics. My main thesis is that visualization is an effective tool for achieving scientific understanding, but – contrary to what some scientists and philosophers have suggested – it is not a necessary condition for understanding. This thesis is embedded in a more general analysis of what scientific understanding consists in, and how it can be obtained (as presented in my book Understanding Scientific Understanding (OUP, 2017). I will support it with a case study of the role of visualization in the genesis of quantum mechanics in the 1920s and in the development of quantum field theory in the years since World War II.