With the increasing application of systematic approaches to biology, new formats are needed for rapidly analyzing protein function on a large-scale. Increasingly, novel types of array are allowing thousands of peptides and proteins to be screened in one experiment, enabling researchers to carry out hypothesis-generating, as well as hypothesis-driven, experiments. In their review on p. 393, Emili and Cagney compare the strength and weaknesses of current array approaches with functional screening of libraries, describe the different ways of arraying peptides, proteins and living cells, contrast the particular strengths of each system, and suggest some potential future applications for the technology. While arrays are unlikely to replace traditional case-by-case studies of protein function, they are likely to prove extremely useful for facilitating the dissection of protein networks.