  The superposition principle is the cornerstone of quantum mechanics, leading
to a variety of genuinely quantum effects. Whether the principle applies also
to macroscopic systems or, instead, there is a progressive breakdown when
moving to larger scales, is a fundamental and still open question. Spontaneous
wavefunction collapse models predict the latter option, thus questioning the
universality of quantum mechanics. Technological advances allow to challenge
collapse models and the quantum superposition principle more and more with a
variety of different experiments. Among them, non-interferometric experiments
proved to be the most effective in testing these models. We provide an overview
of such experiments, including cold atoms, optomechanical systems, X-rays
detection, bulk heating as well as comparisons with cosmological observations.
We also discuss avenues for future dedicated experiments, which aim at further
testing collapse models and the validity of quantum mechanics.
