In this talk, I will present an introduction to a particular enigmatic limit of string theory, the tensionless regime. This is particularly important to understand the very high energy sector of string theory which has been conjectured to display a lot of extra symmetries and holds the key to the mysterious `Hagedorn' phase of strings. I will present some recent findings regarding the quantum structure of such strings and a proposal of open string degrees of freedom emerging from a closed string as the tension is dialed down to zero.