  The configurational entropy is one of the most important thermodynamic
quantities characterizing supercooled liquids approaching the glass transition.
Despite decades of experimental, theoretical, and computational investigation,
a widely accepted definition of the configurational entropy is missing, its
quantitative characterization remains fraud with difficulties, misconceptions
and paradoxes, and its physical relevance is vividly debated. Motivated by
recent computational progress, we offer a pedagogical perspective on the
configurational entropy in glass-forming liquids. We first explain why the
configurational entropy has become a key quantity to describe glassy materials,
from early empirical observations to modern theoretical treatments. We explain
why practical measurements necessarily require approximations that make its
physical interpretation delicate. We then demonstrate that computer simulations
have become an invaluable tool to obtain precise, non-ambiguous, and
experimentally-relevant measurements of the configurational entropy. We
describe a panel of available computational tools, offering for each method a
critical discussion. This perspective should be useful to both experimentalists
and theoreticians interested in glassy materials and complex systems.
