DNA repair can occur by a variety of mechanistically distinct pathways [for review, see (1 )]. Recombinational DNA repair is one such pathway, and it requires the coordinated action of many different enzymes. In the best studied organism, Escherichia coli , more than 20 different proteins are involved [for review, see (2 )]. The recombinational repair of a double-stranded DNA (dsDNA) break requires four general steps: (1) processing; (2) homologous pairing; (3) DNA heteroduplex extension; and (4) resolution. Here we describe assays to study aspects of the first two steps.