The competition between different interactions that cannot be
simultaneously satisfied is often referred to as frustration - a
familiar feature in many magnetic solids. Strong frustration leads to
highly degenerate ground states and a large suppression of ordering.
Key challenges in frustrated magnetism include the characterization of
the fluctuating spin-liquid regime and determination of the mechanism
of eventual order at lower temperature. In this talk, I will discuss a
model of a diamond-lattice antiferromagnet appropriate for numerous
spinel materials. With sufficiently strong frustration, a massive
ground-state degeneracy develops amongst spirals whose propagation
wavevectors reside on a continuous two-dimensional 'spiral surface' in
momentum space. I will argue that an important ordering mechanism is
entropic splitting of the degenerate ground states, an elusive
phenomenon called 'order by disorder'. A broad spiral spin-liquid
regime emerges at higher temperatures, where the underlying spiral
surface can be directly revealed through spin correlations. We discuss
the agreement between these predictions and the well-characterized
spinel MnSc2S4.