When first used in the 18th cent. the term was confined to the study of engravings, which were then the standard mode of illustrating books on art and on antiquities in general. But it came shortly to be applied more specifically to the history and classification of Christian images and symbols of all sorts, in whatever medium they happened to be rendered originally or in whatever way they were reproduced for study.

With the rise of the systematic investigation of art from prehistoric ages to modern times, it became apparent that each major phase or epoch in which figural representations occur had created and developed in varying degrees of richness and elaboration an iconography of its own. As used today, therefore, the term is necessarily qualified to indicate the field of iconographic study under discussion—e.g., the iconography of the various Egyptian deities, the iconography of Roman imperial portraits, early Christian iconography, Buddhist or Hindu iconography, Byzantine iconography, Gothic iconography.

As a method of scholarly research the science of iconography strives also to recover and express the thought from which a given convention of representation has arisen, particularly when the convention has assumed the value of a symbol. The importance of identifying motifs is central to iconographical interpretation. For example, St. Catherine of Alexandria is traditionally portrayed in the presence of a wheel. This wheel is a familiar attribute that serves to identify her and that at the same time signifies a miracle connected with her martyrdom. Some attributes are more difficult to understand, and their obscurity has led scholars to consult other images or literary sources in order to interpret the motif more satisfactorily.

Certain themes characteristic of a specific philosophy have been commonly represented during an era, and an iconography has been developed to express them. An example is the still life vanitas vanitatum of the Middle Ages, a reminder of the transitory quality of earthly pleasure symbolized by a skull, candle, and hourglass (or, in later versions, a watch). In every living art the conventions and symbols, as well as their meanings, change with the passage of time and the growth of ideas; many disappear, while others become almost unintelligible to a later generation and can be recovered only by intensive study. Among the foremost scholars in iconographic studies are Didron, Émile Mâle, Aby Warburg, and Erwin Panofsky.

By reason of its long history and the dynamic concepts that controlled it, the growth of Christian iconography is rich and varied. Beginning with the catacomb frescoes in the early centuries of the Christian era, it deals with the perils faced by the human soul on earth in its journey toward eternal salvation. Figures from the Old Testament (e.g., Abraham, Judith and Holofernes), episodes from the life and passion of Jesus (e.g., the Nativity, the Descent from the Cross, the Pietà), scenes from the life of the Virgin Mary (e.g., the Sacred Conversation, the Visitation), scenes from the lives of the saints (e.g., St. Francis Receiving the Stigmata, St. Jerome in the Wilderness, the Martyrdom of St. Agatha), and symbolic scenes of ultimate beatitude (e.g., the Majesty, the Savior of the World, the Coronation of the Virgin), all reveal the same purpose—to repeat in many forms and inculcate in every mind the moral aims and fundamental dogmas of the Christian religion.

A long series of evolutionary stages unfolds in the representation of a given person or scene from the art of the catacombs to that of the Gothic cathedrals. Thus the art of the Middle Ages is above all a kind of sacred writing whose system of characters, i.e., the iconography, had to be learned by every artist. It was governed also by a kind of sacred mathematics, in which position, grouping, symmetry, and number were of extraordinary importance and were themselves an integral part of the iconography.

From earliest times Christian iconography has likewise been a symbolic code, showing the faithful one thing and inviting them to see in it the figure of another. Some examples are: the dove, which figures the Holy Spirit; the fish, symbol of Christ, from the Greek icthus, an anagram for Jesus Christ, Son of God, Savior; the monkey or reptile as symbol of evil; and the bowl or pitcher of water and the vase of lilies that signify the Virgin's purity in the Annunciation scene. In Christian art, form is thus the vehicle of spiritual meaning; in the expression and reading of this meaning lies the essence of Christian iconography.