Regulation of Respiration In Vivo -- Calorespirometry in Plant Biology -- The Application of the Oxygen-Isotope Technique to Assess Respiratory Pathway Partitioning -- Respiration in Photosynthetic Cells: Gas Exchange Components, Interactions with Photorespiration and the Operation of Mitochondria in the Light -- Effects of Light Intensity and Carbohydrate Status on Leaf and Root Respiration -- The Effects of Water Stress on Plant Respiration -- Response of Plant Respiration to Changes in Temperature: Mechanisms and Consequences of Variations in Q10 Values and Acclimation -- Oxygen Transport, Respiration, and Anaerobic Carbohydrate Catabolism in Roots in Flooded Soils -- Effects of Soil pH and Aluminum on Plant Respiration -- Understanding Plant Respiration: Separating Respiratory Components versus a Process-Based Approach -- Respiratory/Carbon Costs of Symbiotic Nitrogen Fixation in Legumes -- Respiratory Costs of Mycorrhizal Associations -- Integrated Effects of Atmospheric CO2 Concentration on Plant and Ecosystem Respiration. Respiration in plants, as in all living organisms, is essential to provide metabolic energy and carbon skeletons for growth and maintenance. As such, respiration is an essential component of a plant's carbon budget. Depending on species and environmental conditions, it consumes 25-75% of all the carbohydrates produced in photosynthesis - even more at extremely slow growth rates. Respiration in plants can also proceed in a manner that produces neither metabolic energy nor carbon skeletons, but heat. This type of respiration involves the cyanide-resistant, alternative oxidase; it is unique to plants, and resides in the mitochondria. The activity of this alternative pathway can be measured based on a difference in fractionation of oxygen isotopes between the cytochrome and the alternative oxidase. Heat production is important in some flowers to attract pollinators; however, the alternative oxidase also plays a major role in leaves and roots of most plants. A common thread throughout this volume is to link respiration, including alternative oxidase activity, to plant functioning in different environments.