SummaryAn equation of Lighthill's is used to calculate sperm thrusts. They have values in the range 5–350 pN, depending on species. The limitations of this approach are discussed and comparison is made with the measured thrust for human sperm. The effect of sperm thrusts of this magnitude on covalent bonds and reversible bonds is discussed. Sperm cannot break covalent bonds, but can reduce the lifetime of reversible bonds.The structure and physical properties of the zona pellucida are examined in relation to sperm penetration. The evidence suggests that sperm cannot penetrate it solely by force. A model for sperm penetration is elaborated in which the conjunctive application of thrust and a soluble enzyme leads to strain‐induced proteolysis and the formation of the penetration slit. The potential mechanism of the zona block is discussed, as is the site of the acrosome reaction. The effects of other mechanical inputs into fertilization such as stirring and swimming are examined briefly. Evidence suggests that sperm penetration of the cumulus oophorus and cervical mucus is mechanical, but that in the case of cervical mucus, it is affected by changes in the physical properties of the mu