The normal spermatozoon is a differentiated cell with a head and a tail. The head comprises a cap-like acrosome containing enzymes to penetrate the membranes of the oocyte and a well-shaped nucleus. For the spermatozoon to be able to move and fertilize, the tail must be well organized in all its structures, the axoneme, the accessory fibers, the fibrous sheath, and the mitochondria. All sperm components can be altered and therefore functionality compromised. This chapter discusses sperm defects, including their classification into systematic and non-systematic alterations, their potential causes, and the current state of knowledge in this field. New acquisitions are fundamental for increasing therapies and positive outcome in assisted reproduction techniques.