We provide a tutorial on the marine controlled-source electromagnetic (CSEM) method aimed at geoscientists and explorationists who are new to EM methods. This tutorial highlights some of the issues to be considered in planning and executing a CSEM survey, and interpreting the results. CSEM methods can add valuable information on the resistivity structure of the subsurface, which complements measurements obtained from seismic or other geophysical methods. We also discuss the development of the method and provide an overview of the CSEM acquisition approaches applied today. Understanding the sensitivity of the CSEM method to seafloor resistivity structures is key to ensuring a successful survey. We illustrate this with simple examples, demonstrating the effect of reservoir and overburden properties and the effect of electrical anisotropy. It is also important to understand how well a given resistivity structure can be recovered from realistic survey data. We apply an inversion approach to illustrate this for 2D resistivity structures. Finally, we discuss the importance of interpreting CSEM in an integrated framework alongside seismic and well log data.