This chapter details and explains wood anatomy, including the fundamental distinctions between hardwoods, or flowering plants, and softwoods, mainly the conifers. The principal cell types and defining characteristics of secondary xylem are described, with emphasis on the details and traits most relevant to wood identification and work with archaeological wood. The second half of the chapter reviews well-documented variability in anatomical expression that may result from a wide range of internal and external stimuli, including delving further into functional and ecological variation (i.e., juvenile wood, reaction wood, such as were initially described in Chapter 3). The deliberate and secondary effects of human activities influencing wood development and woodland resources generally are also examined.