Aikido testing and ranking is based on the Kyu-Dan system common to many Japanese martial arts.
Students begin as unranked, and after a time, test for 5th Kyu, the first ranked level. As students advance, their rank numbers become smaller (3rd kyu is more advanced than 4th kyu, etc.) until they reach 1st kyu, after which they begin to test for the blackbelt (or Dan) levels, where the numbers increase in size with experience.
Aikido is different from other martial arts in the way that there are no belt colours within the lower ranks. Students continue to wear their whitebelts until the time that they test for their blackbelt. This helps to develop an active and attentive beginners mind as people continue to learn and train towards a more advanced level. It is also in keeping with Japanese tradition, where there were no coloured belts ranks, and you only became a blackbelt after practicing so much and for so long that your original whitebelt would turn black with dirt.
A sample of the testing requirements and minimum practice times are below. Please click on the image for a full, multi-page document spanning the requirements for ranks from 5kyu up to 5th Dan.