An active fluid is a densely packed soft material whose constituent elements can self-propel. Examples include dense suspensions of bacteria, microtubule networks or artificial swimmers. These materials come under the broad category of active matter and differ significantly in properties when compared to passive fluids, which can be described using Navier-Stokes equation. Even though systems describable as active fluids have been observed and investigated in different contexts for a long time, scientific interest in properties directly related to the activity has emerged only in the past two decades. These materials have been shown to exhibit a variety of different phases ranging from well ordered patterns to chaotic states (see below). Recent experimental investigations have suggested tha