Describing and predicting real-world phenomena is an important task for physics. Not only in the microscopic world, but also in the macroscopic world there are still many non-trivial phenomena waiting to be challenged by physical sciences. Active matter is a population of self-propelled particles that convert free energy into motion or force. Examples include flocks of birds, populations of biological cells, cell cytoskeleton, self-driven colloidal particles and vibrated granular rods. Collections of those active particles exhibit solid, liquid and gas like behavior and beyond. A number of deep analogies exist between equilibrium statistical physics systems and active matter systems including phase transitions and macroscopic dynamics. In active matter systems, on the other hand, new macroscopic properties arise. The ordered state of a certain class of cell population closely resembles the order of liquid crystal molecules, but novel phenomena such as spontaneous condensation of cell population into topological defects. may explain the mechanism of morphogenesis in living organisms.