  We argue for the existence of phase transitions in $3+1$ dimensions
associated with the appearance of tensionless strings. The massless spectrum of
this theory does not contain a graviton: it consists of one $N=2$ vector
multiplet and one linear multiplet, in agreement with the light-cone analysis
of the Green-Schwarz string in $3+1$ dimensions. In M-theory the string
decoupled from gravity arises when two 5-branes intersect over a
three-dimensional hyperplane. The two 5-branes may be connected by a 2-brane,
whose boundary becomes a tensionless string with $N=2$ supersymmetry in $3+1$
dimensions. Non-critical strings on the intersection may also come from
dynamical 5-branes intersecting the two 5-branes over a string and wrapped over
a four-torus. The near-extremal entropy of the intersecting 5-branes is
explained by the non-critical strings originating from the wrapped 5-branes.
