In the 15th century, traces of medieval style still lingered in fashion as they did in art. The Renaissance had only just originated in Italy and began to slowly spread to the North of Europe, a process that took all of the 15th century. Next to the rich city states of Italy, Burgundy in France exerted the most influence on a continent that consisted mainly of small fiefdoms that were just too powerless to give rise to sovereigns who could afford to support the arts - arts that could have recorded the fashions of the time.
Thus, most pictorial sources for the 15th century are either French or Flemish (and therefore still medieval Gothic) or Italian (i.e. early Renaissance). The most noted painters (costume-wise) are van Eyck, Memling and Fouquet for the North, da Vinci, Ghirlandaio and Botticelli for the South.
The Renaissance, in contrast to Gothic, is marked by a revival of what was considered classic Greek and Roman. Where art had served almost exlusively for religious (i.e. Christian) purposes, it now dealt with subjects from ancient Greek legend, including gods and goddesses - some of them even (shocking!) naked. In paintings and sculpture, more and more portraits are seen, faces begin to exhibit individuality, postures become more natural, and space becomes three-dimensional by use of perspective.
These changes in art make the 15th century the first that offers proper material for costume research because
However, even the Italian Renaissance painters can not be completely trusted. While the shape of garments, fabric patterns and trimmings are probably accurate, much is still stylised. In paintings by Ghirlandaio, for example, you will notice a certain formalism in a person's posture that very probably affects the folds of the fabric as well. It often is necessary to know a bit about the ostumes of the time to discern actual from fantasy costumes, which often appear side by side in one painting.
Be especially careful when you encounter portraits of Christian saints: Some are depicted in period clothing, some in what the painter considered to be contemporary of the saint. The worst kind wears period costume with certain details that are pure fantasy. Paintings of saints are only included on this site if their dress is contemporary of the painter and looks relatively trustworthy.