  We survey the most important kinds of structural complexity in Prussian blue
analogues, their implications for materials function, and how they might be
controlled through judicious choice of composition. We focus on six particular
aspects: octahedral tilts, A-site `slides', Jahn--Teller distortions, A-site
species and occupancy, hexacyanometallate vacancies, and framework hydration.
The promising K-ion cathode material K$_x$Mn[Fe(CN)$_6$]$_y$ serves as a
recurrent example that illustrates many of these different types of complexity.
Our article concludes with a discussion of how the interplay of various
distortion mechanisms might be exploited to optimise the performance of this
and other related systems, so as to aid in the design of next-generation PBA
materials.
