During the past decade, local force measurement techniques, such as atomic force microscopy and optical tweezers, were used to study the elastic properties and mechanically induced structural transitions of nucleic acids at the single-molecule level. Single-molecule manipulation has also increasingly been used to investigate DNA-dependent enzymatic processes, with implications for unfolding and modifying DNA, protein-DNA interactions, replication and transcription. Compared to classical techniques of molecular biology, single-molecule measurements avoid the need to average over a large number of events, and can thus potentially provide detailed and complementary information.