Adolescent Forerunners, known as Manipulars, were not associated with any particular caste. However, upon maturing, each individual Forerunner would be assigned to a rate, mostly in accordance with the rate of their parents. For example, Bornstellar-Makes-Eternal-Lasting was expected to become a Builder like his father.
Each rate had their own ceremonial headgear, worn only on special occasions. However, on a certain day in the Grand Star Season, every Forerunner wore the same style.
- Main article: Builder
The Builders were the highest caste responsible for the construction of a majority of Forerunner architecture. As such, they were most likely responsible for creating the Halo Array and Onyx. The father of Bornstellar-Makes-Eternal-Lasting was a Builder. Faber held the highest rank of Builder, the Master Builder.
- Main article: Miner
The Miners were the second-highest rate in Forerunner society. They were responsible for obtaining the materials required for the work of the Builders.
- Main article: Lifeworker
The Lifeworkers were the third ranked rate in Forerunner society. They specialized in biology and medicine, especially with the experimentation of various forms of life. It was not uncommon for Lifeworkers to receive their own planets for experimentation. The Librarian held the highest rank of Lifeworker, the Lifeshaper.
- Main article: Warrior-Servant
The Warrior-Servants were the fourth rate beneath all other castes save for the Engineers who acted as the primary military caste in Forerunner society. They were responsible for, among other things, piloting War Sphinxes during the Human-Forerunner war. The Prometheans were the highest ranking, and most elite group of Warrior-Servants. The Didact held the highest rank of Warrior-Servant.
- Main article: Engineer (rate)
The Engineers were the lowest rate in Forerunner society, and were presumably tasked with constructing and maintaining structures and machinery. The highest ranking Engineer was the Plasma jockey.
The Juridicals were an eminent rate during most of Forerunner history, but especially during the final years of the Forerunner-Flood war. It was their role to preside over the highest cases where the Mantle was in danger of being violated. The highest rank of Juridical was the Master Juridical.
The Theoreticals were a scientific rate that was absorbed into the Builder rate long before the Forerunner-Flood war. Little is known about them except that their role appeared to fill both the scientist and historian roles. The only known Theoretical was a female called Boundless.
- Halo: Cryptum (First appearance)
- Halo: Primordium
- Halo: Combat Evolved Anniversary
- Halo: The Thursday War (Mentioned only)
- Halo 4
- Halo: Silentium