In the "male factor" entity, the structural and functional correlation determining the sperm fertilizing capacity is constituted by a group of cellular factors that must be evaluated in the semen of the infertile men. Structurally the spermatozoa of the head, the middle piece and the flagellum. The head has a highly condensed haploid nucleus, surrounded by a thin layer of cytoplasmic material, which is covered in a cap-like fashion by the membrane limiting the acrosome. This last organelle, which has characteristics similar to those of a secretory granule, secretes in a programmed way the hydrolytic enzymes that facilitate the fertilization process. The middle piece contains the mitocondrail sheet, responsible for the energy metabolism of the sperm cell. The flagellum has the same basic structure of other cilia or flagella, but also has particular characteristics due to the presence of the outer dense fibers and the fibrous sheath. In the semen analysis from infertile men the abnormalities most frequently observed belong to the number, morphology, variability, motility of capacitation-acrosome reaction of the sperm cells. However, due to the apparent multifactorial etiology of the male factor, now a days we only have few options for medical or pharmacological treatment. In this paper we review the morphology and ultrastructure considered as "normal" in the human spermatozoa, as well as we describe the most frequent alterations in these parameters. At the same time, we discuss the impact of sperm motility and of the capacitation-acrosome reaction process in male fertility.