15th century Ladies' Gowns

During the 15th century, ladies’ gowns were in transition, from the looser fit of earlier centuries, with an eye toward that coming age of elaborate fashion, the Renaissance.
Typical gowns were still long and full, worn over a kirtle or undergown. A chemise, made of linen or a soft fabric, was worn next to the skin.
Waists became higher, leaving behind the long-waisted look of earlier centuries. A high waist, like our “empire” waist, became popular, with fullness over the stomach.
The outermost gown usually had a V-shaped neck, cut low, the better to show the kirtle beneath. The outer gown could be trimmed with fur or velvet. Sleeves took on more importance and could be highly decorated, with embroidery or jewels. Toward the end of the century, sleeves with slits became popular. The slits enabled the full sleeve of the chemise beneath to be pulled out through the slits, making puffs along the arm, and displaying a contrasting color or fabric. Some sleeves were so elaborate they were transferred from dress to dress. Ladies must have liked the fashion, as elaborate sleeves stayed in fashion for the following two centuries.

Medieval Underwear

Medievalists know very little about underwear worn in the Middle Ages. What we know about clothing comes from the few extant pieces that have survived the years, carefully preserved in museums with controlled climate and lighting, but with underwear—being what it was—we have little to go by. The Chartres statues, for instance, represent outer garments, so we can only guess, from representations on pottery and drawings, at what was worn beneath. Dating from early Rome, there are representations of women participating in games that show them wearing something that looks much like a bikini, a small lower piece and a binding wrap at the top.
When full skirts came into use, it's doubtful women would lift layers of cloth and then have to untie something to answer nature's call, although something like men's loincloths may have been worn during certain times of the month.
Women wore undergowns, or chemises, beneath their outer gowns. In the picture, this woman has her outer gown tucked into her belt, perhaps to allow a bit of air to pass through her chemise, but this was the furthest she'd go.
Men, in early Middle Ages, wore loincloths like what is shown. Laborers in the field thought nothing of stripping down to their loincloths in hot weather. At other times, the clothes were colorful and part of everyday outer garb, as the picture suggests, and men at sea had no compunction about stripping naked during daytime chores on the ship, unless there were women aboard.
We know more about the hose they wore, as that garment is visible in statues and paintings. Hose were made of two woven pieces of fabric sewn together, usually of wool. Their wool was a soft weave because of the manner in which it was made, nothing like our wool today which would be a bit itchy, at least to this writer. Later, hose (hosen) worn by armored knights were made of sturdier material and called chausses, an item worn beneath the armor.
In the late Middle Ages and Early Modern period, hose became a significant part of everyday outer garb and were frequently colorful and made of fine fabrics.
There are several good reference books on the subject, but be careful to steer away from costume books used for Hollywood productions. Some are not true to the period, but look better on screen. A good little overall guide, one I have on my reference shelf and which gives a good idea of the construction of medieval clothing, is Medieval Costume in England and France: the 13th, 14th, and 15th Centuries, by Mary G. Houston. At least it's a place to start your research on this fascinating subject.