Sharding happens on a collection level The sharding key determines how data is distributed db.runCommand( { shardcollection: "demo.cities", key: { name: 1 } } ); Data is stored in chunks, each with a maximum size of 64MB A chunk contains data in the form of a subsection of the shard key *mongos* makes sure that chunks are distributed Chunk meta-data is stored in the *config servers* Each shard is a replica-set