Engineered nucleases are promising tools for genome manipulation and determining off-target cleavage sites of these enzymes is of great interest. This disclosure provides in vitro selection methods that interrogate 1011 DNA sequences for their ability to be cleaved by active nucleases, e.g., ZFNs and TALENs. The method revealed hundreds of thousands of DNA sequences that can be cleaved in vitro by two ZFNs, CCR5-224 and VF2468, which target the endogenous human CCR5 and VEGF-A genes, respectively. Analysis of the identified sites in cultured human cells revealed CCR5-224-induced mutagenesis at nine off-target loci. This disclosure provides an energy compensation model of ZFN specificity in which excess binding energy contributes to off-target ZFN cleavage. It was also observed that TALENs can achieve cleavage specificity similar to or higher than that observed in ZFNs.