The origin of the Kippah

In discussing ritual garb in class tonight, the question was raised as to the origin of the kippah. There are no references in the Torah. In the Talmud the concept is introduced that covering one's head is a way of showing reverance to God. Maimonides ruled that men were to wear kippah during prayers in Mishne Torah. By the mid 16th century, legal codes included the wearing of kippah when out, and even when in one's home.

Here are some resources for more details:
Virtual Jewish Library

Kippot in Jewish Custom