One of the best discriminators for the fertilization potential of human spermatozoa is sperm morphology. The problem in the assessment of the sperm morphological characteristics is their pleiomorphism. Examination of spermatozoa with the light microscope can provide only limited information on their internal structure. More detailed examination of sperm structure using electron microscopy can reveal major, often unsuspected ultrastructural abnormalities. Results and cut-off values for sperm analysis depend on the criteria for normal morphology. World Health Organization recommendations provide a classification suitable for clinical practice. Clinically reliable cut-off limits for normal sperm morphology according to strict Tygerberg criteria were suggested to be 4% in in-vitro fertilization procedures. Patients with severe sperm head abnormalities have a lower chance of establishing successful pregnancies, even though fertilization may be achieved. The outcome of intracytoplasmic sperm injection is not related to any of the standard semen parameters or to sperm morphology. Sperm decondensation defects and DNA anomalies may be underlying factors for the unrecognized derangements of the fertilizing capacity of spermatozoa, regardless of sperm morphology. Centrosome dysfunction may also represent a class of sperm defects that cannot be overcome simply by the insertion of a spermatozoon into the ooplasm. In this article an overview on the composition and ultrastructure of spermatozoa is presented, while emphasizing sperm ultrastructural and sperm DNA anomalies and their effects on fertilization.