Oxygen can fall to low concentrations within plant tissues, either because of environmental factors that decrease the external oxygen concentration or because the movement of oxygen through the plant tissues cannot keep pace with the rate of oxygen consumption. Recent studies document that plants can decrease their oxygen consumption in response to low oxygen concentrations to avoid internal anoxia. This adaptive response involves a restriction of respiration and a concomitant decrease in ATP consumption that results from the inhibition of a wide range of biosynthetic processes. The inhibition of respiration is rapid and occurs at oxygen concentrations well above the K(m)(oxygen) of cytochrome oxidase, indicating that an oxygen-sensing system triggers a coordinated inhibition of ATP formation and consumption. In addition to this, low oxygen concentrations lead to the induction of a plant-specific and energy-conserving pathway of sucrose degradation, which decreases oxygen consumption and improves plant performance. Low oxygen concentrations also lead to long-term morphological adaptations, which allow respiration per volume tissue to be decreased and oxygen entry to be increased. Recently, advances have been made in elucidating possible oxygen-sensing systems and regulatory components that are involved in these responses.