The Excubitors (Latin: excubitores or excubiti, lit. 'those out of bed', i.e. 'sentinels'; transcribed into Greek as ἐξκουβίτορες or ἐξκούβιτοι, exkoubitores/exkoubitoi) were founded in c. 460 as an imperial guard-unit by the Byzantine emperor Leo I the Thracian. The 300-strong force, originally recruited from among the warlike mountain tribe of the Isaurians, replaced the older Scholae Palatinae as the main imperial bodyguards. The Excubitors remained an active military unit for the next two centuries, although, as imperial bodyguards, they did not often go on campaign. Their commander, the Count of the Excubitors (comes excubitorum, κόμης τῶν ἐξκουβίτων), soon acquired great influence. Justin I was able to use this position to rise to the throne in 518, and thereafter the Counts of the Excubitors were among the main political power-holders of their day; two more, Tiberius II Constantine and Maurice, rose to become emperors in the late 6th century.

In the later part of the 7th century the Excubitors appear to have morphed into a parade-ground formation, and they fade from the record as a corps. Individual seals of office suggest that the title of excubitor became an honorific dignity rather than an active military appointment during the early part of the 8th century. This changed c. 760, when the Emperor Constantine V reformed the corps into one of the élite tagmata - professional heavy-cavalry regiments that constituted the core of the Byzantine army of the middle-Byzantine period. Notable members of the regiment during this time include Saint Joannicius the Great (served c. 772 to 792), and Emperor Michael II the Amorian, who served as regimental commander, or Domestic of the Excubitors (δομέστικος τῶν ἐξκουβίτων), before rising to the throne in 820. The Excubitors fought in several campaigns during the following centuries, and are last attested in the disastrous Battle of Dyrrhachium in 1081 that destroyed the remnants of the middle-Byzantine army.

The Excubitors were founded by Emperor Leo I (r. 457–474) in c. 460 and were recruited from among the sturdy and warlike Isaurians, as part of Leo's effort to counterbalance the influence of the magister militum Aspar and the large Germanic element in the East Roman army. Unlike the older palace regiments of the Scholae Palatinae, which were under the control of the magister officiorum and eventually degenerated to parade-ground formations, the Excubitors long remained a crack fighting force.

The unit was headed by the count of the Excubitors (Latin: comes excubitorum; Ancient Greek: κόμης τῶν ἐξκουβίτων/ἐξκουβιτόρων, romanized: komēs tōn exkoubitōn/exkoubitorōn), who was entirely independent of all other officials and subordinated only to the emperor himself. By virtue of his proximity to the emperor, the count of the Excubitors became an official of great importance in the 6th and 7th centuries. This post, which can be traced up to c. 680, was usually held by close members of the imperial family, often virtual heirs apparent. Thus it was the support of his men that secured Justin I (r. 518–527), who held the post at the time of the death of Anastasius I (r. 491–518), his elevation to the throne. Similarly, Justin II (r. 565–578) relied on the support of the Excubitors for his unchallenged accession; their count, Tiberius, was a close friend who had been appointed to the post through Justin's intervention. Tiberius was to be the Emperor's right-hand man throughout his reign, eventually succeeding him as Tiberius II (r. 578–582). He too would be succeeded by his own comes excubitorum, Maurice (r. 582–602). Under Maurice, the post was held by his brother-in-law Philippicus, and under Phocas (r. 602–610) by Priscus. Another powerful occupant was Valentinus, who secured it during the power struggles that accompanied the regency of Empress-dowager Martina in 641, before deposing her and her son Heraklonas and installing Constans II (r. 641–668) as emperor. Valentinus dominated the new regime, but his attempt to become emperor himself in 644 ended in his being lynched by the mob.

By the late 6th century, the count of the Excubitors held the highest court ranks, of patrikios and vir gloriosissimus. Apart from their duties as commander of the Excubitors, holders of the office now also undertook other functions such as recruiting troops and interrogating suspected traitors. The count of the Excubitors was even sent to lead campaigns. The power that went with the position, and the intrigues of men like Priscus and the would-be usurper Valentinus, doomed the post to eventual decline during the latter half of the 7th century, although it is likely that the post continued in existence into the 8th century, until the corps was reorganized.

The participation of the Excubitors in campaigns is well attested, such as in 598, when Emperor Maurice took them to defend the Anastasian Wall against the Avars. They served with Heraclius (r. 610–641) against the Sasanian Persians, and c. 650 some Excubitors appear as guards to Pope Martin I. During the later 7th century, like the Scholae before them, the Excubitors degenerated to a parade-ground unit that saw no active service. Indeed, it appears that during the 7th and early 8th centuries, the titles of excubitor and scribon were awarded as court dignities, paralleling the development of similar formerly military titles such as candidatus. This is evidenced by a large number of seals naming individual excubitors during this time, in stark contrast to the periods before and after, when the Excubitors are attested as an active military unit. Furthermore, in many of these seals, individual excubitores—as well as scribones—are shown to have conjointly held bureaucratic offices, while at least one seal is known of a certain George, who was both excubitor and a scholarius (a member of the Scholae).

After being mentioned in a letter by Justinian II (r. 685–695, 705–711) to Pope John V in 687, the Excubitors as a corps disappear from the historical sources until they re-emerge, under a new commander, the domestic of the Excubitors (δομέστικος τῶν ἐξκουβίτων, domestikos tōn exkoubitōn) and in a new capacity, as one of the imperial tagmata, which comprised the elite professional central army established by Constantine V (r. 741–775) in c. 760. The tagmata were cavalry units, armed and equipped by the imperial arms factories to a higher standard than the provincial ('thematic') forces, likely including horse armour. As such the Excubitors were no longer a palace guard, but a unit actively engaged in military campaigns. At the same time, the tagmata, being loyal to the emperor's person, represented a counterbalance to the thematic armies of the provinces and constituted a powerful tool in implementing the iconoclastic policies pursued by Constantine V. Their original role as palace guardians was taken over by another, newly created tagma, that of the Vigla.

Nevertheless, the possibly first commander of the tagma, Strategios Podopagouros, was among the leaders of a failed plot against Constantine V's life in 765, and was executed after its discovery. This initiated a purge of the new units from suspected opponents of the Emperor's policies. By the 780s, following years of imperial favour and military victories under Constantine V and his son Leo IV the Khazar (r. 775–780), the tagmata had become firm adherents to the iconoclast cause. Within less than two months of Leo V's death in 780, Empress-regent Irene of Athens had to foil an attempt spearheaded by the Domestic of the Excubitors to place Constantine V's exiled second son, Nikephoros, on the throne, and in 785/86 Irene forcibly disarmed them and exiled some 1,500 tagmatic soldiers due to their resistance to the restoration of the icons.