The high cost of deepwater exploration motivated the development of commercial marine magnetotelluric (MT) exploration in 1995, but it wasn’t until marine controlledsource electromagnetic (CSEM) methods burst upon the industry scene with the formation of three new contractors in 2002 that things got really exciting. Now the bubble has burst and the excitement has diminished, but marine EM remains an important tool for offshore exploration. Early mistakes were made as a result of poor instrumentation and a lack of good interpretation tools unlike seismics, EM relies heavily on inversion to produce useful results but both equipment and inversion codes have improved significantly. Still, EM images resistivity, not hydrocarbon content, and false positives occasionally occur, but false negatives are rare. That is, without an EM signature there is little chance of discovering economical hydrocarbons. In this lecture I will review the history, discuss the 10 important things you need to know about marine EM, and look to the future of the method.