Esoteric Knowledge remains a central problem of ancient science, since much of the scholarly heritage of ancient schooling was only meant for a small circle of adherents, rather than for the general public. For this reason, knowledge was restricted within schools or within professions, only accessible to those with a personal connection to experts and savants. The expression ‘esoteric’ is used today rather indiscriminately. Within the category of ‘esoteric knowledge’ one understands a variety of related expressions, such as ‘mystical’ or ‘occult’, as well as the more concrete ‘absolute’ or ‘elevated’ knowledge, which can also be considered as ‘hidden’, ‘secret’, or ‘inaccessible’, and even ‘fanciful’ or carried away. The confused pattern of these definitions is the reason why a closer look at the historical development of this concept is so important. This Preprint provides case studies of esoteric knowledge, within the realms of philosophy (e.g. esoteric vs. exoteric knowledge), religious knowledge (early Christian thought, Gnosticism, cultic practices), geography, divination, alchemy, dream interpretation, and iconography. Examples of esoteric knowledge presented here extend from Mesopotamia and Egypt into Graeco-Roman and early medieval models, in Akkadian, Egyptian, Syriac, Greek and Latin sources, indicating the durability and continuity of this concept.