The hair follicle is subject to a constant turnover in the course of perpetual cycles through phases of proliferation, involution, and resting, with regeneration in the successive hair cycle. Understanding the basics of the hair cycle enables insight into the principles of hair growth and shedding. Many factors can lead to a pathologically increased hair loss. Whatever the cause, the follicle tends to behave in a similar way. To grasp the meaning of this generalization requires understanding the varied derangements of the normal hair cycle. Cyclic hair growth activity occurs in a random mosaic pattern with each follicle possessing its own individual control mechanism over the evolution and triggering of the successive phases, including the local milieu at the level of the stem cells. In addition, a number of systemic and environmental factors may have influence, such as hormones, cytokines and growth factors, toxins, and deficiencies of nutrients, vitamins, and energy (calories). Normal supply, uptake, and transport of proteins, calories, trace elements, and vitamins are of fundamental importance in tissues with a high biosynthetic activity such as in the course of hair cycling. It may appear that on a typical Western diet, people are not subject to nutritional deficiencies. Nevertheless, genetic diversity in nutrient requirements, inappropriate food selection or preparation, intensive physical exertion, comorbidities, and use of drugs may lead to deficiency symptoms resulting in unhealthy hair. In fact, nutritional needs fluctuate with age and with situations that occur throughout the life cycle: infancy, childhood, adolescence, pregnancy, lactation, old age, lifestyle (restricted diets, smoking, alcohol consumption), and health status (chronic disease, medications).