  Topological insulators constitute one of the most intriguing phenomena in
modern condensed matter theory. The unique and exotic properties of topological
states of matter allow for unidirectional gapless electron transport and
extremely accurate measurements of the Hall conductivity. Recently, new
topological effects occurring at Dirac/Weyl points have been better understood
and demonstrated using artificial materials such as photonic and phononic
crystals, metamaterials and electrical circuits. In comparison, the topological
properties of nodal lines, which are one-dimensional degeneracies in momentum
space, remain less explored. Here, we explain the theoretical concept of
topological nodal lines and review recent and ongoing progress using artificial
materials. The review includes recent demonstrations of non-Abelian topological
charges of nodal lines in momentum space and examples of nodal lines realized
in photonic and other systems. Finally, we will address the challenges involved
in both experimental demonstration and theoretical understanding of topological
nodal lines.
