Oscillations play an important role in many dynamic cellular processes. They can emerge as the collective dynamic behavior of an ensemble of interacting components in the cell. Examples include oscillations in cytoskeletal structures such as the axonemes of cilia. Spontaneous oscillations of mechano-sensitive hair bundles have been shown to give frequency selectivity and amplification to mechano-sensation. In some bacteria, oscillations of Min proteins are important for division site selection. Genetic oscillators form the basis of circadian clocks. All these oscillations share many general features. Models and theoretical approaches are essential for an understanding of the principles underlying these dynamic cellular processes.