When you complete missions your ranks will be increased and you will receive promotions, Metal, and Supplies, you will also get experience which is useful for leveling up. If you find a particular mission hard to complete there is alwayas the 'Bypass' option which will skip the mission and replace it with another one. Note: You can only recieve 3 missions at once. It is important that you remember to upgrade your storage buildings when you can as this will allow you to constantly keep aquiring more loot from the various missions you go out on.
Missions have two categories of Objectives, the first are regular Objectives which reward you for reaching certain points in the game, upgrading parts of your base to certain levels, or if you manage to kill a certain number of zombies. The second type of Objectives are Daily Objectives which reset each and every day, allowing you an almost endless way to receive and acquire new shipments of Metal and Supplies.