Two-dimensional (2D) supramolecular self-assembly of various organic molecules at the liquid-solid interface is presented and discussed with a focus on compounds that are primarily functionalized by carboxylic groups. The main analytical tool utilized is scanning tunneling microscopy (STM), a high-resolution real-space technique capable of readily providing full crystallographic information (i.e., not only lattice parameters but also number, type, and orientation of molecules within the unit cell). Carboxylic groups are of particular interest because their combined donor and acceptor character with regard to hydrogen bonds provides reliable intermolecular cross-linking, thereby facilitating the self-assembly of well-ordered, stable monolayers. By means of various homomeric (monomolecular) and heteromeric (here, bimolecular) examples, this feature article illustrates the influence of both molecular structure and external conditions type of solvent, concentration, etc.) on monolayer self-assembly at the liquid-solid interface. A very intriguing aspect of interfacial self-assembly is that many systems are thermodynamically controlled (i.e., adsorbed molecules at the surface are in equilibrium with molecules dissolved in the supernatant liquid phase). This offers the unique possibility not only to steer the system reliably by intensive thermodynamic parameters Such as temperature and concentration but also to gain fundamental knowledge about decisive processes and steps in supramolecular self-assembly.