Identification of an insect from midwest US
Brown marmorated stink bug (Halyomorpha halys):



See here, here, here or here for information.

These have become hugely invasive in the Northeast and Mid-Atlantic (and slowly Southeast) portions of the U.S. as well as along the West Coast. 

The brown marmorated stink bug (Halyomorpha halys) is native to Eastern Asia, and likely 1st started spreading around the U.S. in Pennsylvania in the early 2000s.



Finally, you can see an image of the life stages of H. halys here. 

Update:

Just for clarification, other brown stink bugs do exist and can be easily confused with H. halys. This article from UF IFAS, for example, demonstrates how to differentiate H. halys from Euschistus servus:



See here and here for additional ID tips and further reading.