Iron-based superconductors are famous for a close interplay of structure, magnetism and superconductivity. A central theme of research is their tetragonal-to-orthorhombic structural transition. It is strongly coupled to stripe-type antiferromagnetism in most - but not all - systems. FeSe is a prominent example here. There is a consensus that this displacive phase transition is of electronic origin. The corresponding electronic degree of freedom was termed nematic, alluding to the liquid-crystal phase. In addition to the lattice distortion, the nematic transition causes clear features in the electronic properties. However, a more specific probe of nematic order is the electronic anisotropy between the orthorhombic a- and b-axes, which is prominent in basically all experimental quantities. The existence of an electronic nematic order parameter, irrespective of its microscopic origin, means that there is a related susceptibility. Over the last decade, multiple experimental approaches to the nematic susceptibility have been developed. Among other things, they provide experimental evidence for an electronic driving mechanism of the nematic transition.

"Nematic" transitions have by now been reported in an increasing number of materials. However, iron-based superconductors are ideal systems to study this electronically-driven lattice distortion as its signatures are pronounced, it can be investigated with a wide range of experimental techniques, it is easily tunable, and high-quality single-crystal samples are widely available for these systems. Intriguingly, nematic fluctuations appear favorable for superconductivity, but the interplay of nematicity and superconductivity is still largely unknown.