  Our intuitive understanding of cosmic structure formation works best in
scales small enough so that bound, relaxed gravitating systems are no longer
adjusting their radius; and large enough so that space and matter follow the
average expansion of the Universe. Yet one of the most robust predictions of
$\Lambda$CDM cosmology concerns the scale that separates these limits: the
turnaround radius, which is the non-expanding shell furthest away from the
center of a bound structure. The maximum possible value of the turnaround
radius within the framework of the $\Lambda$CDM model is, for a given mass $M$,
equal to $(3GM/\Lambda c^2)^{1/3}$, with $G$ Newton's constant and $c$ the
speed of light, independently of cosmic epoch, exact nature of dark matter, or
baryonic effects. We discuss the possible use of this prediction as an
observational test for $\Lambda$CDM cosmology.
