  This review discusses picosecond ultrasonics experiments using ultrashort
hard x-ray probe pulses to extract the transient strain response of
laser-excited nanoscopic structures from Bragg-peak shifts. This method
provides direct, layer-specific, and quantitative information on the picosecond
strain response for structures down to few-nm thickness. We model the transient
strain using the elastic wave equation and express the driving stress using
Gr\"uneisen parameters stating that the laser-induced stress is proportional to
energy density changes in the microscopic subsystems of the solid, i.e.,
electrons, phonons and spins. The laser-driven strain response can thus serve
as an ultrafast proxy for local energy-density and temperature changes, but we
emphasize the importance of the nanoscale morphology for an accurate
interpretation due to the Poisson effect. The presented experimental use cases
encompass ultrathin and opaque metal heterostructures, continuous and granular
nanolayers as well as negative thermal expansion materials, that each pose a
challenge to established all-optical techniques.
