We analyzed the atomic models of 75 X-ray structures of protein-nucleic acid complexes with the aim of uncovering common properties. The interface area measured the extent of contact between the protein and nucleic acid. It was found to vary between 1120 and 5800 A2. Despite this wide variation, the interfaces in complexes of transcription factors with double-stranded DNA could be broken up into recognition modules where 12 +/- 3 nucleotides on the DNA side contact 24 +/- 6 amino acids on the protein side, with interface areas in the range 1600 +/- 400 A2. For enzymes acting on DNA, the recognition module is on average 600 A2 larger, due to the requirement of making an active site. As judged by its chemical and amino acid composition, the average protein surface in contact with the DNA is more polar than the solvent accessible surface or the typical protein-protein interface. The protein side is rich in positively charged groups from lysine and arginine side chains; on the DNA side the negative charges from phosphate groups dominate. Hydrogen bonding patterns were also analyzed, and we found one intermolecular hydrogen bond per 125 A2 of interface area in high-resolution structures. An equivalent number of polar interactions involved water molecules, which are generally abundant at protein-DNA interfaces. Calculations of Voronoi atomic volumes, performed in the presence and absence of water molecules, showed that protein atoms buried at the interface with DNA are on average as closely packed as in the protein interior. Water molecules contribute to the close packing, thereby mediating shape complementarity. Finally, conformational changes accompanying association were analyzed in 24 of the complexes for which the structure of the free protein was also available. On the DNA side the extent of deformation showed some correlation with the size of the interface area. On the protein side the type and size of the structural changes spanned a wide spectrum. Disorder-to-order transitions, domain movements, quaternary and tertiary changes were observed, and the largest changes occurred in complexes with large interfaces.