Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect. 4. Nonmagnetic designs 54 C. Toward even higher dimensions 55 1. Synthetic dimensions 55 2. Four-dimensional quantum Hall effect 56 VI. Gain and loss in topological photonics 56 A. Non-Hermitian topological photonics 56 B. Emergent topology of Bogoliubov modes 57 VII. Topological effects for interacting photons 58 A. Weak nonlinearities 59 B. Strong nonlinearities 60 VIII. Conclusion and perspectives 62 A. Optical isolation and robust transport 62 B. Quantum emitters and topological laser 63 1. Topological lasers: Theory 63 2. Topological lasers: Experiments 63 C. Measurement of bulk topological and geometrical properties 64 D. Topological quantum computing 65 Acknowledgments 65 References 66