The illuminator, who was a specialist distinct from the scribe, had a repertoire of visual motifs which he, or she, employed to decorate the manuscript according to the nature of the text and the expense of the commission. Letters that began new chapters or important passages in the book could be decorated or historiated. Decorated letters were embellished with geometric, foliate, and zoomorphic designs, or with mixed elements of all three. Historiated initials, deriving their name from the French "ystoire", served as frames that enclosed small figural or narrative scenes.

To further enliven the text, the margins of the page were often adorned with decorated borders. Their decoration varied from small line drawings of a whimsical character, known as "drolleries," to elaborately painted floral patterns filling the entire border. In some instances, small scenes were incorporated into the border in the form of medallions called roundels or rectangular panels in the lower margin.

For more expensive commissions, paintings known as miniatures were often included in the decorative program. Miniatures are named not for their small size but from the Latin word "minum", which is a red pigment used in paint. Miniatures enhanced the beauty of the book with narrative and symbolic scenes. Their functions ranged from illustrating the text and dividing the book into sections, to serving as devotional icons and aids to study and prayer. Within this context, a diverse range of regional and personal styles developed; making each manuscript unique in both style and content.

The paint used to decorate manuscripts and paint miniatures came from a variety of sources including oxidized metals as well as vegetable and animal matter in a tempera base. Vermilion was made from mercury and sulfur, while ultramarine blue, a pigment as expensive as gold, was made from crushing lapis lazuli, a semi-precious stone imported from Afghanistan during the Middle Ages. Materials were very expensive, and sometimes substitutes for real gold were used.

Before the thirteenth century, medieval manuscripts were initially produced in monasteries by monks working in the scriptorium, or writing room where books were made. More than five hundred monasteries existed in England alone by the twelfth century, and a typical monastic library might possess over three hundred books in its library. By the beginning of the thirteenth century, the growth of towns and the establishment of universities in Paris, Oxford, and Bologna led to the rise of secular scribes and artists who served students and professors as well as the nobility.

Later in the fourteenth century, a rise in literacy and the development of an upper-middle-class created a large demand for illuminated manuscripts. The production of illuminated manuscripts of prayer books for personal devotions, called Books of Hours, increased dramatically.