  The string equivalent of a massless particle ($m=0$) is the tensionless
string ($T=0$). The study of such strings is of interest when trying to
understand the high energy limit of ordinary strings. I discuss the classical
$T\to 0$ limit of the bosonic string, the spinning string and the superstring.
A common feature is the appearence of a space-time (super-)conformal symmetry
replacing the world-sheet Weyl invariance. The question of whether this
symmetry may survive quantization is addressed. A lightcone analysis of the
quantized bosonic tensionless string leads to severe constraints on the
physical states: they are space-time diffeomorphism singlets characterized by
their topological properties only.
