The aliens' choice of UFOs is limited by their research and resources, with the actual ship sent on a mission being chosen randomly - with equal probability for each - from the available pool. All ten mission types are listed and discussed below, followed by a table detailing the scheduling of regular missions.
There are also two special mission types that occur irregularly. The type of each mission is determined by the resources the aliens have, and how threatening XCOM is perceived to be. There are eight types of regular alien missions scheduled on a monthly basis.
In other words, an alien mission always starts with an UFO sortie. Aliens will dispatch sorties of UFOs to further their goals during the war, and XCOM may respond in kind by sending out interceptors and the Skyranger to foil their missions in the air or on the ground. During the long war against the alien threat, the skies are also part of the battlespace.
