He was known for his researches on the period from the 4th through 11th centuries with a special interest in the advance of Islam, interactions with religion and thought, and military subjects.
[3] He was known also as the co-founder of the Byzantine Studies Conference and the editor of the journal Byzantinische Forschungen.
Kaegi was most recently involved in several projects, notably on Muslim raids into Byzantine Anatolia.
Kaegi's research interests also included Byzantine commercial relationships with the Arabian Peninsula on the eve of the Islamic conquests.
Additionally, he was preparing an essay on Byzantium in the 7th century for an Oxford University Press handbook to Maximus the Confessor.