This thesis presents the first full study of the anonymous illuminator known by the name of convention, the Master of Spencer 6, after his finest work, ms. 6 in the Spencer Collection at the New York Public Library. Active at the turn of the sixteenth century, during the transitional period between the Middle Ages and the Renaissance, this artist provides a revealing case study for examining the changing tastes and preoccupations of the patrons, as well as the way in which illuminators were operating in order to secure work and forge a career. The career of the Master of Spencer 6 can be reconstructed from nearly forty surviving books and fragments. He appears to have painted manuscripts for a wide range of clientele, from unknown merchants to figures such as Henry VII of England. The quality of his execution is equally varied, from modest, hastily prepared images, to exquisite paintings invested with verisimilitude and invention that deserve wider acknowledgement. This illuminator, presumably based in Bourges, seems to have travelled as far as Troyes, Paris, Tours, and possibly Lyon, in search of patronage. Although he specialised in devotional images, he also illustrated texts of historical and moral interest. The Master of Spencer 6 was particularly talented in drawing. He appears to have been required to work quickly, in order to respond to the high demand for books; yet, despite the haste, he was able to produce images that were pleasing. A large part of the appeal in his images seems to rely on the quality of line. While his colours were clean and bright, he often applied them hastily or carelessly over the contour lines. Nearly always these shortcomings appear unnoticeable due to the beauty of the lines that define the design. The variety of decorative schemes, layouts, spatial devices, compositions and iconographic motifs utilised by the Master of Spencer 6 demonstrate one of the keys to his success. He was able to diversify his canon to realise any potential order from the vast geographical and social range of his clientele. He also managed to develop his style according to current tastes and fashions. He adapted ideas and techniques from his collaborators, the Colombe workshop and Jacquelin de Montluçon. This thesis provides also the first study of Jacquelin de Montluçon, the painter identified here as the main collaborator of the Master of Spencer 6. Methods of technical art history are used to analyse his sophisticated manner of mixing pigments to produce convincing effects of light. The way in which he applied paint onto a surface, on parchment, panel and stained glass, is used to support attributions and explore the versatile artist that emerges from the analysis. This investigation into these two hitherto little-known artists demonstrates, on one hand, what was required for artists to succeed over others in the profession of manuscript illumination in late fifteenth-century France, and on the other hand, what the concerns of the individuals commissioning images were.