Xylem is one of the two types of transport tissue in vascular plants (phloem is the other). The word xylem is derived from the Greek word ξύλον (xylon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant. Its basic function is to transport water, but it also transports some nutrients through the plant.

The most distinctive xylem cells are the long tracheary elements that transport water. Tracheids and vessel elements are distinguished by the multitude of spots that occur on the vessel elements. In these spots both outer and inner walls are missing. The vessel elements are connected together into long tubes that are called vessels.

Xylem also contains other cell types, for example parenchyma.

Xylem can be found:

In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only.

The branching pattern exhibited by xylem follows Murray's law.

Primary xylem is the xylem that is formed during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. It is distinguished by wider vessels and tracheids. Xylem development occurs in a number of patterns, which vary in the relative position of the protoxylem and metaxylem, e.g. endarch in which the protoxylem is towards the centre of the stem or root, or exarch in which the metaxylem is towards the centre.