  The judicious design of electromagnetic boundary provides a crucial route to
control light-matter interactions, and it is thus fundamental to basic science
and practical applications. General design approaches rely on the manipulation
of bulk properties of superstrate or substrate and on the modification of
boundary geometries. Due to the recent advent of metasurfaces and
low-dimensional materials, the boundary can be flexibly featured with a surface
conductivity, which can be rather complex but provide an extra degree of
freedom to regulate the propagation of light. In this perspective, we denote
the boundary with a non-zero surface conductivity as the meta-boundary. The
meta-boundaries are categorized into four types, namely isotropic, anisotropic,
biisotropic and bianisotropic meta-boundaries, according to the electromagnetic
boundary conditions. Accordingly, the latest development for these four kinds
of meta-boundaries are reviewed. Finally, an outlook on the research tendency
of meta-boundaries is provided, particularly on the manipulation of
light-matter interactions by simultaneously exploiting meta-boundaries and
metamaterials.
