Active matter is an extreme kind of non-equilibrium system in that detailed balance is broken at the microscopic scale. A typical active system can be a collection of particles that continuously exert mechanical forces on their surrounding environment, and systems of interacting active particles exhibit diverse emergent behaviours. But how do we tell whether the behaviours observed are novel to physics? One way to do so is to determine whether the emergent phenomena are described by novel universal classes. This is the approach I will take in this talk. Specifically, I will start by deriving the hydrodynamic equations of motion for active fluids from symmetry consideration, and then discuss the associated universal behaviours at criticality and in the ordered phase. I will show that some of these universal behaviours can be mapped onto existing equilibrium models, while others are novel or potentially novel to non-equilibrium physics.