The concept of risk in the field of avitaminoses is very important and useful for the practitioner, who should consider two aspects: a) risk factors, which could be individual (physiological, pathological and psychological) and extra-individual (alimentary, environmental, etc.); b) subjects with an elevated risk of avitaminosis (childhood, old age, pregnancy, etc.). In these subjects the risk can be a generical one, when there is an elevated requirement for all vitamins (nursing women, sportmen, etc.) or a specific one, when there is a high requirement only for a single vitamin (osteomalacia, some professional diseases, use of oral contraceptives) or a vew of them (alcoholism, diabetes, etc.). On the basis of this kind of knowledge it is easy for the practitioner to estimate which vitamins are necessary for each subject or for a group of subjects in physiological or in pathological conditions. For example, there is an elevated risk of apyridoxinosis in old age (acalciferolosis in aged women), of athiaminosis and apyridoxinosis in diabetes, of apyridoxinosis in oral contraceptives users, of axeroftolosis in hyperthyroidism, of athiaminosis, apyridoxinosis, aniacinosis and anascorbosis in alcoholics. In the second chapter the concept of the latency period in avitaminosis is illustrated. This period corresponds to the interval between the moment when deficiency stimulus starts operating and the moment when its effect, that is the picture of avitaminosis, appears. The latency time is not measurable, on account of the difficulties in establishing the onset of the deficiency stimulus; generally it is very long and is followed by the period of biochemical symptomatology and subsequently by the one of clinical symptomatology. Each of these three phases can be further divided in several steps, which have summarized in a Table. The last chapter is dedicated to the classification of avitaminoses. From the etiopathogenetic point of view avitaminoses can be due to: a) deficiency of introduction (alimentary level)); b) deficiency of absorption (enteric level); c) deficiency of utilization (tissue level). From the clinical point of view avitaminoses can be distinguished in deficiency with: a) a complete clinical symptomatology (scurvy, beriberi, pellagra, rickets, osteomalacia, xerophthalmia, hemeralopia); b) an incomplete clinical symptomatology (mono- or oligo-symptomatic or partial clinical picture); c) a biochemical symptomatology only (subclinic or clinically asymptomatic picture).