The ability of cells to explore their environment is important for a range of biological systems, including bacteria, algae and spermatozoa. The independent motion of cells is known as cell motility, and allows for cells to seek out nutrients, follow chemical signals, and escape danger. In the case of bacteria, motility is involved in cell propagation and proliferation. The standard method of measuring motility involves microscopic tracking of individual cells, so only a finite number of cells can be tracked, and obtaining statistically reliable measurements of populations is time consuming and difficult. Differential Dynamic Microscopy (DDM) is a relatively new technique in which a time-evolving sample is imaged at fast frame rates, and the intensity fluctuations in each pixel are correlated. DDM complements dynamic light scattering and has been applied to a wide range of systems including characterisation of colloidal nanorods, as well as the measurement of motility in bacteria and algae, and allows the measurement of statistically valid velocity distributions. I will explain the principles of DDM, illustrates with examples of the characterization of bacterial motility, and discuss the advantages and disadvantages of the technique.