  Planar, disordered assemblies of small particles incorporated in layered
media -- sometimes called ``disordered metasurfaces'' in the recent literature
-- are becoming widespread in optics and photonics. Their ability to scatter
light with exotic angular and spectral features in reflection and transmission,
as well as their suitability to scalable fabrication techniques, makes them
promising candidates for certain applications, ranging from thin-film
photovoltaics to visual appearance design. This chapter introduces the basic
concepts and theoretical models for the specular (a.k.a. coherent) reflectance
and transmittance of electromagnetic waves by disordered metasurfaces. After
describing the classical scattering formalism for discrete media, we establish
known analytical expressions for the reflection and transmission coefficients
of disordered particle monolayers on layered substrates. Two classical models,
based on the independent scattering approximation (ISA) and the effective field
approximation (EFA), are presented. Their accuracy is examined by comparing
predictions with those obtained from rigorous full-wave computations using an
in-house multiple-scattering code. This chapter may serve as a starting point
to students and researchers who wish to dive into the topic and explore the
potential of disordered metasurfaces for applications.
