Zinc finger domains (ZnFs) are common, relatively small protein motifs that fold around one or more zinc ions. In addition to their role as a DNA-binding module, ZnFs have recently been shown to mediate protein:protein and protein:lipid interactions. This small zinc-ligating domain, often found in clusters containing fingers with different binding specificities, can facilitate multiple, often independent intermolecular interactions between nucleic acids and proteins. Classical ZnFs, typified by TFIIIA, ligate zinc via pairs of cysteine and histidine residues but there are at least 14 different classes of Zn fingers, which differ in the nature and arrangement of their zinc-binding residues. Some GATA-type ZnFs can bind to both DNA and a variety of other proteins. Thus proteins with multiple GATA-type fingers can play a complex role in regulating transcription through the interplay of these different binding selectivities and affinities. Other ZnFs have more specific functions, such as DNA-binding ZnFs in the nuclear hormone receptor proteins and small-molecule-binding ZnFs in protein kinase C. Some classes of ZnFs appear to act exclusively in protein-only interactions. These include the RING family of ZnFs that are involved in ubiquitination processes and in the assembly of large protein complexes, LIM, TAZ, and PHD domains. We review the similarities and differences in structure and functions of different ZnF classes and highlight the versatility of this fold.