Gas hydrates are solid, ice-like mixtures of water and low molecular weight hydrocarbons. They are found under the permafrost and, to a far greater extent, under the seafloor. Hydrates are a potential energy resource, a possible factor in climate change, and a geohazard for offshore drilling operations. Because of their ice-like nature, hydrates are electrically insulating. It follows that a method which remotely detects changes in seafloor electrical resistivity, such as the transient marine controlled source electromagnetic (CSEM) method, is a useful geophysical tool to map and assess hydrate deposits. In this talk, I describe the fundamentals of marine CSEM with an emphasis on gas hydrate exploration.