It is widely accepted that siege warfare formed a significant part of medieval conflict, and one in which cavalry played only a minor role. This article argues that horsemen were in fact essential to the prosecution of a successful siege, and crucial for an effective defense. Cavalry could prevent an army from setting up a siege but also ward against that threat. Once a siege began the horsemen of both sides contested the struggle for supply, both in the vicinity of the siege, and also on its periphery. Cavalry was the most dangerous arm for a sally, but also the key to a successful response, and a vital component of a relieving force. Cavalry dominated the numerous skirmish actions that took place around a siege and was also the arm of pursuit, an army retreating from a failed investment being highly vulnerable to horsemen. Horses drove mills, carried provisions, pulled the artillery, and were present at sieges in considerable numbers, making them instrumental in the spread of disease due to the large amounts of waste they produced. However, their role in the transmission of bacillary dysentery, the great scourge of medieval siege warfare, was probably other than by water-borne infection. War horses were valuable animals and siege warfare offered opportunities for their capture, while its privations caused the animals great distress, not least because they required considerable maintenance. In good condition horses contended with the enemy and secured supplies but, if provisions failed, their excessive needs rapidly made them weak and ineffective, with drastic consequences both for themselves and their human counterparts. An analysis of several sieges during the later Hundred Years War examines these issues and considers whether the English were compromised by the number and quality of their mounted men-at-arms.