The obligation to provide men, weapons and equipment for local defence is an old one and can be traced back to the Anglo-Saxon period when able-bodied males were liable for military service. The Anglo-Saxon system was augmented by later laws. The Statute of Winchester of 1285 decreed the weapons which every man must keep according to his assessed wealth and ordered that an inspection of arms and equipment take place twice a year. The Statute was confirmed by Henry VIII in 1511 and was only superseded in 1558 by the Act for the keeping of Arms and Horses. Men were assessed in ten groups according to income, ranging from those worth £5-10 a year who provided a coat of plated armour, a bill or halberd, longbow and steel helmet, to men worth £1000 or more and obliged to provide 16 horses, 80 suits of light armour, 40 pikes, 30 longbows, 20 bills or halberds, 20 arquebuses and 50 steel helmets.

By the sixteenth century the county forces were usually only mustered together when there was a threat of foreign invasion or local unrest. Otherwise, the central government Council ensured that men and equipment were kept in a state of readiness by ordering muster certificates to be prepared by the counties. The orders were sent to the head of the county authorities, the sheriff and selected gentlemen, who served as commissioners of musters. The orders were then relayed to the high constables of the hundreds, who also received assessments of men, equipment and money and passed these to the parish constables. Once the muster was complete, a certificate was sent by the muster commissioners to the Council.

Muster lists for the year 1539 survive for several counties including some of the earliest known Bedfordshire lists, those for Bedford town and the hundreds of Barford, Stodden, Willey, Manshead, Flitt and Redbornstoke. There is a general correlation between the dates of surviving muster lists and rebellion and foreign wars, but some lists also survive for comparatively peaceful times. In 1539 Emperor Charles V of the Holy Roman Empire and Frances I of France were intending to mount a joint expedition to invade England and punish Henry VIII for his breach with Rome. The whole of England's military strength was mobilised; the levies in the north to man the Scottish border while those of the south were to face the French.