Xylem and phloem transport processes in trees are classical problems in plant physiology, but still somewhat weakly understood. The transport of water from the soil to the leaves is especially intriguing since it occurs under metastable conditions where water pressure has decreased below saturation vapour pressure. This often leads to the formation of gas bubbles (embolisms) and disruption of xylem transport. Embolisms have also been shown to refill with water, but mechanisms for refilling remain uncertain. In this book these plant physiological problems are addressed as purely physical processes. Also the osmotically driven translocation of sugars in the phloem is analysed with a whole tree level model. It is predicted that phloem transport is tightly coupled to xylem water potential and hindered by a rise in phloem viscosity when phloem sap becomes highly concentrated.