Synths, created by the mysterious Institute, are classified into one of three generations depending upon their complexity:
- Generation 1 synths are purely mechanical humanoids with skeletal appearances; consequently, they are crude representations of living humans.
- Generation 2 synths are also mechanical, but have more advanced frames that are partially or fully covered in plastic panels that serve as "skin"; they might pass for humans if not closely inspected.
- Generation 3 synths represent the pinnacle of synth technology; with biomechanical bodies capable of respiration, bleeding, consuming, and feeling emotions and sensations, Generation 3 synths are indistinguishable from true humans. It is not uncommon for Gen 3 synths to be unaware that they are a synth.
Many denizens of the Commonwealth have become paranoid and prejudiced against synths, fearing they might be abducted by the Institute and replaced with identical copies of themselves. The Institute and Brotherhood of Steel both see synths as machines only and not as living things or deserving of any rights. The Institute uses them as free labor. The Brotherhood of Steel believes their human-like traits and forced service to the Institute makes them abominations of nature that must be destroyed. The Railroad is the only group with any sympathy for synths. They see the Gen 3s as equals to their fellow man. Since Gen 3s can think for themselves, make choices, and feel emotion such as pride, sorrow, or even love, the Railroad sees the Synths forced labor and servitude within the Institute as slavery.