Purpose of Review This review shows that a more or less constant rate of tip-to-base vessel widening across species, together with the assumption that wider vessels are more vulnerable to embolism, suggests how climate should limit maximum vegetation height; together, these two factors predict a maximum mean vessel diameter permitted by temperature and water availability at a site and thus maximum plant height. Recent Findings Empirical work makes it increasingly clear that the main driver of variation in mean vessel diameter is plant size, specifically the length of the conductive stream. Anatomical evidence, together with hydraulic optimality models, suggests that this vessel diameter-stem length relationship is the result of natural selection favoring the maintenance of constant hydraulic resistance over size increases. From their very narrow termini, vessels widen predictably from the stem tip to the base, following approximately a power law, i. e., with very rapid widening toward the tips and nearly constant diameter toward the base. This size dependence must be taken into account when studying the hydraulics-climate relationship. Summary This review discusses outstanding predictions that require testing, including the following: variation in the vessel diameter-stem length relationship should involve factors such as vessel length distributions, pit characteristics, leaf area, and wood density; leaves higher in trees should have higher terminal leaf vein-petiole base vessel widening rates; species without "disposable" units (e. g., columnar cacti) might have different widening rates; and within-plant widening rate should vary as plants approach their height limits. Finally, we emphasize the need to standardize for size in making comparisons of vessel diameter variation.