Centrosomes of human gametes need to be characterised in more detail, since it was recently shown in 1991 that the human embryo inherits the dominant paternal centrosome at fertilization. Thus there has been a resurgence of centrosomal research in the last decade. The sperm centrosome, containing a single centriole, is preserved and dominant while the egg centrosome is reduced and inactivated during oogenesis, since there should be only one functional centrosome to ensure normal development. This presentation reviews the ultrastructure of gamete centrosomes and attempts to define their respective roles in assisted fertilization. Human testicular and ovarian tissues, sperm, eggs and zygotes were examined by routine TEM. Developing sperm cells and oogonia have two functional centrioles (diplosomes) in their centrosomes, showing the typical "9+0" organisation of microtubule triplets--common to somatic cells. The diplosomes are surrounded by pericentriolar material (PCM), which nucleate microtubules (MT) that organize the cytoskeleton and mitotic spindles. During spermiogenesis, when the spermatids transform into mature sperm, there is a partial reduction of the male centrosome, during which the proximal centriole (PC) is retained intact in the sperm neck, proximal to the nucleus, while the distal centriole (DC) which gave rise to the sperm flagellum, is partially reduced and merges distally with the sperm axoneme in the midpiece and tail. The proximal functional centriole is located in a "black box" in the neck, composed of the capitulum beneath the basal plate and flanked laterally by 9 segmented columns and shows the typical "9+0" organisation of triplets of MT. The PC contains dense material both within and outside the triplets, and the central doublet of axonemal MT terminates in a clump of dense material below its lower vault. The DC is disorganised proximally showing loss of triplets and cannot function as a typical centriole, since the central doublets of axonemal MT traverse through it. Oogonia present a pair of well-defined centrioles, which are involved in cell division. These are lost during oogenesis and the mature oocyte is devoid of centrioles, as in most mammals. Neither does the human oocyte have granular centrosomal material at meiotic spindle poles, in contrast to mouse oocytes which have a dominant maternal centrosome. Thus the oocyte centrosome is greatly reduced and inactivated. Functional centrosomal structure is, however, restored after fertilization in the zygote with some maternal input around the sperm centriole, which duplicates at the pronuclear stage, forms a sperm aster and proceeds to form the first mitotic spindle. This is the ancestor of centrosomes in embryonic, foetal and adult somatic cells. In 1991 (Sathananthan, 1991), we postulated that sperm centrosomal dysfunction could lead to aberrant embryonic development based on centriolar defects in sperm with impaired motility. This hypothesis has now gained acceptance and further evidence to support this theory of infertility is presented.