In the following discussion, we will mostly be concerned with scalp hair. Although scalp and body hair share many of the same characteristics, it is balding that we seek to understand and remedy, so this will be the focus unless otherwise indicated. Hair is a living entity, although the shafts we see are largely the extension, outside the body, of a dead protein called keratin. Beneath the level of the skin, however, there sits a powerhouse of cellular activity. This cell division and differentiation pushes the hair shaft outward; thus we see hair growth that averages about one half inch per month. There are two types of hair: terminal and vellus. The terminal hairs are the thick, strong, usually pigmented ones that constitute the greatest volume of our hair. The vellus hairs are the very fine, short, lightly pigmented hairs, sometimes called "baby hairs", that can be often detected around the hairline, and throughout the scalp, as well as the light "fuzz" elsewhere on the body. This difference is significant when discussing balding, for in balding, the terminal hairs gradually undergo a metamorphosis called "miniaturization". They become shorter, finer, and less colored; in short, they become vellus hairs. If you look closely at the heads of balding men, you will see that, in many of the areas of balding, the strong terminal hairs are replaced by these fine, vellus, "baby" hairs; this is miniaturization, and we will discuss this in greater detail in the next section on hair loss. In addition to these two types of hairs, the individual characteristics of the hair are very important in determining appearance (including the appearance of balding) and styling options. These characteristics include color, curl or kink, caliber (cross-sectional area) and density (number of hairs per unit area). For example, although African hair tends to have a lower density (hairs per square inch) than Caucasian hair, its extreme curliness lends it a great volume and ability to achieve coverage after hair transplant surgery. Asian hair, although it tends to be very coarse (high caliber), may be more challenging to transplant due to the straight nature of the hair, and the contrast between dark hair and lighter skin. However, these are but a few examples, and all of these characteristics are taken into account by the hair restoration expert in designing a program for each unique individual. This is where the art of hair restoration meets the science. In addition to the hair shafts themselves, there are other structures involved in the growth and function of hair. Each hair is associated with a sebaceous gland, which produces the oily sebum that serves to lubricate the hair shafts. Likewise, there is an erector pili muscle, which contracts to raise the hair in situations of anger, fright or cold. Although this function serves little purpose in humans, we see it in other mammals when their "hackles" rise. Hair growth and rest occurs in three phases: anagen, catagen and telogen. The anagen phase is the period of active growth. At any time, about 85 to 91% of hairs are in this active, anagen phase. Anagen may last from 2 to 7 years, but usually about 3 years on average. The catagen phase is considered transitional, and the telogen phase is the dormant, or resting phase. When the hair enters the telogen stage, the hair is either pulled out during normal grooming, or is pushed out when the new hair shaft growth begins. Last, but certainly not least, is the anatomical and physiological unit we have come to call "follicular units". If you have done any current reading on hair restoration surgery, you have surely come across this term. Believe it or not, it was only in the early 1980’s that it was recognized that hairs grew not as strictly single shafts, but as discrete groups of one, two, three, four, and rarely five terminal hairs. These hairs are joined by one or two vellus hairs, a sebaceous gland, an erector muscle, a neurovascular bundle (meaning the nerve and blood vessels), and the perifolliculum, or fine connective tissue sheath. These units are easy to see emerging from the skin under slight magnification on cross section through the skin (figure 1) or when the hair is clipped short (figure 2).