Our chromosomes experience a constant barage of DNA damaging agents that can induce double-stranded DNA breaks (DSBs) - long considered one of the most dangerous forms of DNA damage. Failure to correctly repair DSBs can lead to oncogenesis or cell death. Our cells avoid these consequences by employing an error-free repair pathway called homologous recombination (HR) that can regenerate the continuity of broken chromosomes by using homologous DNA as a template for replication and replacement of the damaged regions. Many of the proteins involved in homologous recombination are conserved throughout evolution, as are the biochemical mechanisms - highlighting the importance of this repair pathway. For example, the bacterial recombinase RecA shares ~30% sequence identity with its human homolog Rad51, and both proteins share very similar biochemical and physical characteristics. Despite many years of intensive study, many details of HR remain unknown. We are studying the molecular mechanism of homologous recombination by making fluorescent proteins and using TIRFM to watch the proteins as they interact with their DNA substrates. This apporach has the potential to reveal mechanistic details that simply can not be detected with standard ensemble measurements. For example, we have shown that human Rad51 can slide along double-stranded DNA via a one-dimensional random walk driven solely by thermal energy (Brownian motion). This is the first experimental evidence showing that Rad51 can move on DNA, and is also the first (and only!) experimental approach that allows the direct observation of 1D-diffusion of proteins on DNA.