The PHRAP assembly program provides rapid comparison, alignment, and assembly of large sets of DNA sequences. PHRAP compares sequences by searching for pairs of perfectly matching "words" or sequence regions that meet certain criteria. If a match is found, PHRAP then tries to extend the alignment into overlapping sections called contigs. PHRAP uses quality values produced by the PHRED basecaller to strike a balance between tolerance of discrepancies and prevention of stacking repeat sequences. The PHRAP assembly algorithm is generally used as part of the PHRED/PHRAP/Consed software suite for sequence analysis. This unit presents instructions for basic usage of the PHRAP assembler, including preparation of the input files (Support Protocols 1 and 2) and explanation of output files (Basic Protocols 1 and 2). Several command line options for changing the PHRAP assembly parameters are also discussed (Basic Protocol 3).