The book will describe the xylem structure of different plant groups, and will put the findings in a physiological and ecological context. For instance, when differences in vessel diameter are featured, then there will be an explanation why this matters for water transport efficiency and safety from cavitation. The focus is on the hydraulic function of xylem, although mechanical support and storage will also be covered. Featured plant groups include ferns (which only have primary xylem), conifers (tracheid-based xylem), lianas (extremely wide and long vessels), drought-adapted shrubs as well as the model systems poplar and grapevine. The book chapters will draw on the expertise and cutting edge research of a diversified group of internationally known researchers working in different anatomical and physiological sub-disciplines. Over the last two decades, much progress has been made in understanding how xylem structure relates to plant function. Implications for other timely topics such as drought-induced forest dieback or the regulation of plant biomass production will be discussed.