The occurrence of membrane vesicles along the male reproductive tract and in the ejaculated semen appears as a common feature among different species, including humans. Indeed, prostasomes (prostate-derived vesicles) were first described in human semen in 1978 (Ronquist et al, 1978), and vesicular structures similar to prostasomes were also found in the seminal plasma of rabbit (Davis, 1978), ram (Breitbart and Rubinstein, 1982), and stallion (Arienti et al, 1998; Minelli et al, 1998). Furthermore,‘‘prostasome-like’’particles are present in the epididymal fluid of rat (Fornes and De Rosas, 1991), hamster (Yanagimachi et al, 1985), and bull (Frenette and Sullivan, 2001) and are also secreted by the bull seminal vesicles (Agrawal and Vanha-Perttula, 1987). The biochemical composition of prostasomes purified from human semen has been well documented. These are multilamellar lipoprotein membrane particles with a diameter of 50 to 500 nm. Their cholesterol-phospholipid ratio reaches 2, sphingomyelin being the major phospholipid. Many proteins are associated with prostasomes, some of them having a catalytic activity (Review: Kravets et al, 2000; Ronquist and Nilsson, 2002). This review will focus on the physiological implication of these membranous structures with regard to the posttesticular sperm maturation.