  Iron-based superconductors are well-known for their intriguing phase
diagrams, which manifest a complex interplay of electronic, magnetic and
structural degrees of freedom. Among the phase transitions observed are
superconducting, magnetic, and several types of structural transitions,
including a tetragonal-to-orthorhombic and a collapsed-tetragonal transition.
In particular, the widely-observed tetragonal-to-orthorhombic transition is
believed to be a result of an electronic order that is coupled to the
crystalline lattice and is, thus, referred to as nematic transition. Nematicity
is therefore a prominent feature of these materials, which signals the
importance of the coupling of electronic and lattice properties.
Correspondingly, these systems are particularly susceptible to tuning via
pressure (hydrostatic, uniaxial, or some combination). We review efforts to
probe the phase diagrams of pressure-tuned iron-based superconductors, with a
strong focus on our own recent insights into the phase diagrams of several
members of this material class under hydrostatic pressure. These studies on
FeSe, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and
CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ were, to a significant extent, made possible by
advances of what measurements can be adapted to the use under differing
pressure environments. We point out the potential impact of these tools for the
study of the wider class of strongly correlated electron systems.
