  Colloidal gels constitute an important class of materials found in many
contexts and with a wide range of applications. Yet as matter far from
equilibrium, gels exhibit a variety of time-dependent behaviours, which can be
perplexing, such as an increase in strength prior to catastrophic failure.
Remarkably, such complex phenomena are faithfully captured by an extremely
simple model - "sticky spheres". Here we review progress in our understanding
of colloidal gels made through the use of real space analysis and particle
resolved studies. We consider the challenges of obtaining a suitable
experimental system where the refractive index and density of the colloidal
particles is matched to that of the solvent. We review work to obtain a
particle-level mechanism for rigidity in gels and the evolution of our
understanding of time-dependent behaviour, from early-time aggregation to
ageing, before considering the response of colloidal gels to deformation and
then move on to more complex systems of anisotropic particles and mixtures.
Finally we note some more exotic materials with similar properties.
