  Efros-Shklovskii (ES) Coulomb gap in density of localized states and ES law
of the variable range hopping conductivity were coined 50 years ago. The theory
and its first confirmations were reviewed in our monograph 40-years ago. This
paper reviews the subsequent experimental evidence, theoretical advancements,
and novel applications. Out of hundreds of experimental validations of the ES
law in a diverse range of materials, I focus on those where the dynamic range
of conductivity exceeds four orders of magnitude. These include three- and
two-dimensional semiconductors under both zero and high magnetic fields,
localized phases in the quantum Hall effect, granular metals, nanocrystal
arrays, and conducting polymers. Additionally, this review discusses the
Coulomb gap near insulator-metal transition and in non-ohmic ES law. Recent
developments of other concepts of the ES book are also discussed.
