Wesen (pronunciation: VAY-zin, Grimm: VES-sin; Germ. "a being", here "creature") is a term used to describe the creatures visible to the Grimms. Wesen are liminal, that is, they display two states of existence simultaneous within one physical body. ("Happily Ever Aftermath") Physiologically, Wesen are parahuman, exhibiting two distinct sets of DNA within the same system. ("Big Feet") When in human form, Wesen are, by all accounts, physiologicaly that way and thus can past for humanity. Every creature encountered by Nick Burkhardt belongs to the Wesen.
The community consists of both good and evil creatures ranging from Blutbaden to Bauerschwein to Jägerbar. Individually, Wesen behave stereotypically in accordance to their own kind (i.e. the Blutbaden being fierce and vindictive, Siegbarste being dangerously vengeful, or Klaustreich known for being opportunistic and shrewd).
Though it cannot be certain, it appears that ingesting Grimm blood can turn Wesen into regular humans.
It may also be possible to turn Wesen into normal humans via drugs. Konstantin Brinkerhoff pioneered a series of medicines that would, supposedly, remove the ability to Woge. While the experiments were undoubtably a failure ("Big Feet"), it must be pointed out that Brinkerhoff insisted that the drugs would work if he'd been given the time to perfect the dosage and delivery system.
Since Nick discovered his identity as a Grimm, he has encountered the following creatures belonging to the Wesen community: