| Abstract: | In this talk I will present the theory of interplay between different macroscopic orderings universally found in iron based superconductors. In most of iron pnictides nematic order is a byproduct of stripe magnetism. In FeSe, the nematic transition is not followed by magnetism at ambient conditions. I will present a unifying description of both of these two different scenarios. Next I'll present the parquet Renormalization Group (RG) study of the interplay between different ordering tendencies. The RG flow of susceptibilities favors the nematic instability even though the magnetism is the only interaction channel with attraction at the bare level. In most of iron pnictides the Fermi energy is large and RG is not capable of tipping the scale in favour of structural transition. Hence, the leading instability is magnetic or superconducting. In FeSe the small Fermi energy allows for RG flow to run long enough to promote the nematic instability, and at the same time to suppress magnetism. |