In 2008, Haldane and Raghu proposed that unidirectional electromagnetic states analogous to (integer) quantum Hall states can be realized in nonreciprocal magnetic photonic crystals. This prediction was first realized in 2009 in the microwave frequency regime. This was followed by the proposals for analogous quantum spin Hall states of electromagnetic waves that are now known as photonic topological insulators. It was later found that topological electromagnetic states can exist in continuous media as well--theoretical and numerical study has confirmed the existence of topological Langmuir-cyclotron waves in continuous magnetized plasmas.