Life Model Decoys (LMD) are robots designed to be indistinguishable from ordinary humans in both behavior and appearance.
It has been mentioned by Dr. Radcliffe that the orginal LMD program was started by S.H.I.E.L.D. and he created his own LMD, AIDA, using S.H.I.E.L.D.'s orginal research. Furthermore, in "Hot Potato Soup", it was mentioned Billy and Sam Koenig were both technicians on the program.
Originally, the brains of LMDs were cybernetic, but after AIDA read the Darkhold she developed a way to create what Radcliffe called a "quantum brain"; a brain made entirely out of extradimensional quantum energy that functions almost exactly like a real human brain.
By using the Framework, Radcliffe developed a method of copying a subjects mind and then downloading all of that subjects memories and personality into an LMD imposter. This way the LMD wouldn't just look like the person it's replacing, but act like them and think like them as well.
In relation to this, Radcliffe was able to create an LMD version of May who believed she was the real Melinda May. Radcliffe explained how May's LMD's programming acts on subconscious level thus creating a subconscious desire to get close to Coulson and learn where the Darkhold is. The May LMD only realized she was a robot after she was injured in battle and saw metal and wires under her skin.
In "The Man Behind the Shield," AIDA replaced Mace, Mack, Fitz, and Coulson with LMD imposters, but unlike the May LMD the other LMD's knew they were robots. This was due to AIDA not having enough time to make their programming subconscious like with May's.
In "World's End," Fitz constructed an LMD of Simmons which AIDA confuses for the real Simmons and destroys in order to hurt Fitz.
In "Self Control", after Ivanov becomes critically injured from a fight with Daisy, AIDA creates a LMD of Ivanov and downloads his mind into it. However, he is controlling his LMD body remotely; AIDA decapitated Ivanov and preserved his head in a jar after attaching wires to it that allow him to control his LMD body.
Currently there are several Ivanov LMD's operating, each of them being remotely controlled by the human brain of the original Ivanov. As long as Ivanov's human brain is unharmed, he can continuously build more LMDs of himself.
- In "Self Control", Daisy knew Simmons wasn't an LMD because when Daisy used her vibration powers on Simmons she could tell Simmons' bones were human bones and not made of metal by the way they vibrated.