Hydrodynamic or collective effects dominate the modern dynamical picture of a heavy ion collision. We discuss the origin, evolution and detection of local and event-by-event anisotropic fluctuations in that picture. We argue that interesting information about the initial state, the early time dynamics and the medium properties can be extracted from measuring multi-particle correlation functions. To that end it is important to separate collective flow from other correlations as the arise for example from resonance decays. We discuss and compare different methods to do that putting special emphasis on the recently developed analytic multi-particle correlation technique (Q-cumulants).