Marine and hydrokinetic energy can be broadly categorized as wave energy, tidal current, open-ocean current, river current, and ocean thermal gradient. Salinity gradient energy is often included as a form of marine energy. The technologies used to convert these forms of energy to electricity are often referred to collectively as marine hydrokinetic (MHK) energy technologies. This paper will give a brief introduction to MHK technologies and their current status. It will also explore the scientific, technical, and nontechnical challenges and barriers to the wide-spread use of MHK technologies. The challenges and barriers include: 1) siting and permitting barriers, 2) environmental impact research needs, 3) technical research and development issues, 4) policy issues, 5) market development barriers, 6) economic and financial issues, 7) grid integration barriers, and 8) education and workforce training needs.