The chapter begins with the descendants of Aaron and Moses. Aaron's sons are Nadab, Abihu, Eleazar and Ithamar, consecrated as priests. Nadab and Abihu died before the Lord when they offered unauthorized fire in the wilderness of Sinai, and they had no sons; so only Eleazar and Ithamar served as priests with their father.
The Lord commands that the tribe of Levi be brought before Aaron to assist him and to care for the tabernacle. The Levites are given to Aaron and his sons as helpers, but only Aaron's family may serve as priests; any outsider who comes near will die. God explains that he has taken the Levites for himself in place of every firstborn of Israel, because the firstborn have been his since the night he struck the firstborn of Egypt.
The Levite males are counted from one month old, by clans. The Gershonites camp on the west, the Kohathites on the south and the Merarites on the north, while Moses, Aaron and his sons camp on the east, in front of the entrance. Each clan receives its charge over the tabernacle and its parts. Then Israel's firstborn are counted: there are 273 more of them than there are Levites, and the Lord commands that this surplus be redeemed by paying five shekels each, money Moses gives to Aaron and his sons.