This article offers a study of the lance and its evolution as a heavy cavalry weapon in the late medieval period. The development of the arrêt de cuirasse, a device upon which the heavy lance depended, dramatically increased the force of the strike, but considerably complicated the process of couching. The resultant loss of cohesion caused tactical difficulties, making the weapon both potent and problematic. Spanish and English men-at-arms often preferred the lighter, more versatile lancegay, which proved useful in the pursuit and for engaging those on foot. An analysis of the performance of heavy cavalry in several engagements suggests that smaller units of skilled men-at-arms were more successful than larger bodies of horsemen containing inexperienced individuals. The criteria necessary for an effective charge are examined, including the effects of weather and terrain, particularly in relation to the contest between the longbow and the mounted man-at-arms. Stakes appear to have been an unreliable anti-cavalry defence, and, in the right circumstances, could be penetrated, knocked over, or avoided in a well conducted charge. Small elite groups of experienced cavalry were able to exploit the considerable power of the lance while minimizing its disadvantages, eventually giving the French an edge over their less flexible, infantry-centered English opponents. The final French victory in the Hundred Years War is often attributed to artillery, but heavy cavalry played a significant role.