A Battlefleet comprises the vessels responsible for protecting a sector of the Imperium. A battlefleet is commanded by a Lord Admiral and is the largest operational naval organisation, usually consisting of between 50 and 75 ships of varying size. Depending on the importance or enemy threat level of a sector this number may be more or less.
A battlefleet is made up of destroyers, frigates, cruisers, and battleships, as well as countless small vessels such as transports, shuttles, messenger craft and long-range patrol craft. In addition there will also be numerous non-warp travel ships such as system patrol ships and defense monitors. These are all backed up by stationary defenses such as space stations, orbital defense platforms, orbital mines, and ground-based defense lasers and missile silos.
A battlefleet's roles include routine patrols, protecting merchant shipping from pirates and alien attack, transporting and escorting Imperial Guard, giving orbital support for planetary armies and providing vessels for exploration fleets.
Each battlefleet is divided down into a number of Battlegroups. The majority of battlegroups are not permanent but are assembled to perform particular functions along the lines of task forces, convey escorts or patrol flotillas.