Active matter is an umbrella term which applies to a broad range of living and synthetic systems, composed of individual units each capable of consuming energy to perform motion, hence driving the system out of thermodynamic equilibrium. Dense ensembles of cells, in the form of two-dimensional confluent tissues, are at the basis of morphogenesis through the formation of the germ layers, and so form an important cross-disciplinary application of these principles. We consider two general classes of theoretical models to describe such ensembles. First self-propelled particles, which are individuals extracting momentum from a substrate in the form of a stochastic force field, and which are used to represent e.g. crawling MDCK cells. We note however that some tissues, such as gastrulating avian embryos, develop in the absence of a substrate from which they can extract momentum. Therefore, we introduce a second model of active tissues driven by internal active stresses, such as the ones which emerge from the contractile activity of the cytoskeleton, and experiencing external dissipation. In all cases we aim to uncover how the microscopic symmetries of the active driving transfer to the macroscopic behaviour of the system. In self-propelled systems we show the emergence of remarkable velocity correlations, and explore how this may drive active turbulent flows. In tissues with active stresses, we show the emergence of two-dimensional long-range structural order and hyperuniformity. These results are based on analytical computations and numerical simulations of disk particles as well as vertex models. We conclude with discussions on the applications of these principles to the analysis of epithelial cell movements.