  Optical communication is an integral part of the modern economy, having all
but replaced electronic communication systems. Future growth in bandwidth
appears to be on the horizon using structured light, encoding information into
the spatial modes of light, and transmitting them down fibre and free-space,
the latter crucial for addressing last mile and digitally disconnected
communities. Unfortunately, patterns of light are easily distorted, and in the
case of free-space optical communication, turbulence is a significant barrier.
Here we review recent progress in structured light in turbulence, first with a
tutorial style summary of the core concepts, before highlighting the present
state-of-the-art in the field. We support our review with new experimental
studies that reveal which types of structured light are best in turbulence, the
behaviour of vector versus scalar light in turbulence, the trade-off of
diversity and multiplexing, and how turbulence models can be exploited for
enhanced optical signal processing protocols. This comprehensive treatise will
be invaluable to the large communities interested in free-space optical
communication with spatial modes of light.
