The structure of solid–liquid interfaces is critical for a large range of systems, including electrochemical energy conversion and storage, corrosion control, water purification, biological signal transduction, and more. However, our understanding of these interfaces remains highly limited. Existing theory and experimental tools have shed light on the structure of solid surfaces; in contrast, the configuration and distribution of molecular species on the liquid side has been largely elusive. The problem becomes more challenging in realistic systems where the solid surfaces inevitably contain various heterogeneities, such as vacancies, step edges and surface corrugations. In this talk, I will discuss our recent efforts on experimental and theoretical investigations of liquid structure at solid–liquid interfaces. Using in situ 3D atomic force microscopy and surface-sensitive vibrational spectroscopy, we have experimentally determined the structure of a diverse set of solvents and electrolytes at flat, homogeneous and rough/heterogeneous solid surfaces. Furthermore, we have developed a statistical mechanics-based model to both explain the experimental results and predict liquid structures near solid surfaces with arbitrary morphologies. At the end, I will discuss how the interfacial liquid structure can modulate electrochemical reaction dynamics, including nucleation and growth processes and electrocatalytic reactions.