Abstract: Tensionless string theory is a candidate for the ultra high energy limit of string theory. In this talk I discuss some of the scenarios when a string with tension becomes tensionless based on our earlier work. Thereafter I revisit the formulation of tensionless closed bosonic string theory following earlier works where it will be revealed that there can be three different quantum theories for tensionless strings. Afterwards I discuss our latest work where we have studied all the three quantum theories in a compactified target spacetime to study the impact of compactification on all these theories.