Abstract. I will consider four-dimensional N=1 effective field theories (EFTs) including strings and membranes, of the kind arising from string/M-theory compactifications. I will describe a general relation between the existence of fundamental axionic strings, dubbed EFT strings, and infinite distance limits in the EFT field space. These limits can be interpreted as classical RG flows along which the EFT strings become tensionless. This highlights the existence of universal structures characterising the perturbative regimes of 4d N=1 EFTs coupled to gravity, which are directly connected with various swampland criteria. In particular, the Swampland Distance Conjecture is reproduced from a bottom-up perspective. Furthermore, the analysis of a large class of string/M-theory models points at a universal pattern relating the mass of the microscopic asymptotically lightest states and the tension of the EFT string associated with the infinite distance limit.