Severe flooding can lead to partial or complete submergence of the shoot, which is an adverse situation triggering a number of responses by the plant, like the formation of aquatic adventitious roots. These roots may not reach the soil surface but instead remain floating in the floodwater. Aquatic adventitious roots partly replace the function of the soil roots as sites for nutrient and water uptake but can also play a role in oxygen exchange with the floodwater and even produce functional chloroplasts to contribute to the oxygen and carbon balance of the plant. This chapter discusses the functions and regulation of aquatic adventitious roots as a response to submergence with key examples from wild wetland plants and rice.