Transient complexes are crucial for diverse biological processes such as biochemical pathways and signaling cascades in the cell. Here, we give an overview of the transient interactions; the importance of transient interactions as drug targets; and the structural characterization of transient proteinprotein complexes based on the geometrical and physicochemical features of the transient complexes' interfaces. To better understand and eventually design transient proteinprotein interactions (TPPIs), a molecular perspective of the proteinprotein interfaces is necessary. Obtaining high-quality structures of proteinprotein interactions could be one way of achieving this goal. After introducing the association kinetics of TPPIs, we elaborate on the experimental techniques detecting TPPIs in combination with the computational methods which classify transient and/or non-obligate complexes. In this review, currently available databases and servers that can be used to identify and predict TPPIs are also compiled.