The dynamics of a wide range of biological systems such as deformable cell layers, epithelial cells, bacterial colonies, and subcellular filaments can be described by the physics of active nematics. I will describe the main concepts used to build active nematic models and then highlight some of our work on how activity can build up orientational order, and lead to a state with continuous creation and annihilation of defects (locations in which the direction of cells is not well-defined). Finally, I will discuss some of our latest results on the role of activity in transitions from a 2D biological layer to 3D structures.