  We study superconformal and supergravity models with constrained superfields.
The underlying version of such models with all unconstrained superfields and
linearly realized supersymmetry is presented here, in addition to the physical
multiplets there are Lagrange multiplier (LM) superfields. Once the equations
of motion for the LM superfields are solved, some of the physical superfields
become constrained. The linear supersymmetry of the original models becomes
non-linearly realized, its exact form can be deduced from the original linear
supersymmetry. Known examples of constrained superfields are shown to require
the following LM's: chiral superfields, linear superfields, general complex
superfields, some of them are multiplets with a spin.
