<p>Metal halide perovskites are swiftly becoming a leading class of photovoltaic materials, yet unlike conventional semiconductors, they exhibit highly anharmonic lattice vibrations that produce extreme rates of thermal expansion and substantial mismatch with other device layers. These effects are exacerbated under natural day/night cycling, where repeated heating and cooling drive cumulative stress, defect generation and accelerated degradation. Here we connect the atomistic origins of anharmonic lattice dynamics in perovskites with their macroscopic thermo-mechanical properties, from phonon–phonon interactions and local disorder to complex thermal–phase relations. We assess how both the degree of anharmonicity and thermal expansion rates vary across temperature, composition and phase, and highlight the emergence of anisotropic and even negative thermal expansion in low-symmetry perovskite phases. By linking fundamental physics to device-level challenges, we provide a framework for engineering durable perovskite absorbers and outline promising approaches to regulate thermal strain in the pursuit of long-lived, commercially viable solar cell technologies.</p>