The heterotrimeric enzyme complex RecBCD is a molecular machine that initiates repair of double-stranded DNA breaks in Escherichia coli by homologous recombination. The enzyme, known to be one of the fastest helicases, is made up of two helicase motors of opposite polarity. It also contains a potent nuclease domain. These activities are modulated upon recognition of a cis-acting DNA element, known as Chi, resulting in the enzyme’s recombination-promoting function. Structural studies using X-ray crystallography have been carried out to understand the working of the enzyme complex. This has led to insights into the mechanism of DNA unwinding and translocation by the bipolar helicase. Furthermore, the molecular basis of directionality of translocation, a fundamental aspect of helicases, has been proposed.