Missions are the name given to Mass Effect's 2 Main quests. Completing Missions advance the story forward and most of the time rewards the player with experience points, Credits and/or Morality Points.
All Missions in Mass Effect 2
Main Missions
Main Missions are the core Missions of the game, focusing on the battle against the Collectors and the Reapers.
Dossiers
Dossiers are Missions you take on to recruit members to the Normandy crew
Dossier: Archangel
Dossier: The Convict
Dossier: The Professor
Dossier: The Warlord
Dossier: Tali
Dossier: The Assassin
Dossier: The Justicar
Dossier: The Master Thief
Dossier: The Veteran
Loyalty Missions
Loyalty Missions are tackled to obtain the loyalty of one of your squadmates, this unlocks an additional power for said teammate as well as having a huge influence on the outcome of the final mission.
Garrus: Eye for an Eye
Grunt: Rite of Passage
Jack: Subject Zero
Jacob: The Gift of Greatness
Legion: A House Divided
Miranda: The Prodigal
Mordin: Old Blood
Samara: The Ardat-Yakshi
Tali: Treason
Thane: Sins of the Father
Zaeed: The Price of Revenge
