  Limited bonding valence, usually accompanied by well-defined directional
interactions and selective bonding mechanisms, is nowadays considered among the
key ingredients to create complex structures with tailored properties: even
though isotropically interacting units already guarantee access to a vast range
of functional materials, anisotropic interactions can provide extra
instructions to steer the assembly of specific architectures. The anisotropy of
effective interactions gives rise to a wealth of self-assembled structures both
in the realm of suitably synthesized nano- and micro-sized building blocks and
in nature, where the isotropy of interactions is often a zero-th order
description of the complicated reality. In this review, we span a vast range of
systems characterized by limited bonding valence, from patchy colloids of new
generation to polymer-based functionalized nanoparticles, DNA-based systems and
proteins, and describe how the interaction patterns of the single building
blocks can be designed to tailor the properties of the target final structures.
