Mass spectrometry combined with affinity purification techniques has evolved as a prime tool for the in-depth study of distinct protein complexes and protein-protein interactions. It fueled proteome-wide studies leading to the establishment of intricate cellular protein interaction networks. Recent innovative advancements in quantitative protein mass spectrometry act as driving force for the design of ingenious strategies in interaction proteomics facilitating the acquisition of interaction data with improved accuracy and, most intriguingly, the elucidation of functional aspects by monitoring transient interactions as well as dynamic changes in composition, stoichiometry, localization and post-translational modification of protein complexes under various conditions.