In mass spectrometry-based proteomics, frequently hundreds of thousands of MS/MS spectra are collected in a single experiment. Of these, a relatively small fraction is confidently assigned to peptide sequences, whereas the majority of the spectra are not further analyzed. Spectra are not assigned to peptides for diverse reasons. These include deficiencies of the scoring schemes implemented in the database search tools, sequence variations (e.g. single nucleotide polymorphisms) or omissions in the database searched, post-translational or chemical modifications of the peptide analyzed, or the observation of sequences that are not anticipated from the genomic sequence (e.g. splice forms, somatic rearrangement, and processed proteins). To increase the amount of information that can be extracted from proteomic MS/MS datasets we developed a robust method that detects high quality spectra within the fraction of spectra unassigned by conventional sequence database searching and computes a quality score for each spectrum. We also demonstrate that iterative search strategies applied to such detected unassigned high quality spectra significantly increase the number of spectra that can be assigned from datasets and that biologically interesting new insights can be gained from existing data. Molecular & Cellular Proteomics 5:652-670, 2006.Proteomics, the systematic identification and characterization of all proteins expressed in a cell, has become a key analytical approach in the life sciences (1). The dramatic progress of proteomic research over the last decade has been catalyzed by several, seemingly independent developments. First, the wealth of genomic sequence information generated by large scale sequencing projects and the development of computational gene prediction and annotation tools have produced sequence databases that are expected to contain most coding gene regions. These databases can be searched with proteomic data and constrain the proteomic search space (2). Second, technological improvements in mass spectrometry and peptide and protein separation techniques allow rapid and sensitive protein identification from minute amounts of complex biological samples (for reviews, see Refs. 1, 3, and 4). Third, the development of computational tools for the assignment of MS/MS spectra to peptide sequences and the statistical validation of these assignments support the consistent analysis of large datasets with no or minimal human intervention (5). Collectively these developments resulted in the emergence of shotgun proteomics, a strategy based on the combination of tandem mass spectrometry-based peptide sequencing and sequence database searching, which now routinely permits the identification of hundreds to thousands of proteins in a single experiment.Shotgun proteomics creates significant computational challenges (5-8). Large numbers (on the order of 10 5 ) of MS/MS spectra acquired in each experiment need to be computationally processed to identify peptides that produced them and to infer what proteins were present i...