What have we learned from observational cosmology ?
  We review the observational foundations of the $\Lambda$CDM model, considered
by most cosmologists as the standard model of cosmology. The Cosmological
Principle, a key assumption of the model is shown to be verified with
increasing accuracy. The fact that the Universe seems to have expanded from and
hot and dense past is supported by many independent probes (galaxy redshifts,
Cosmic Microwave Background, Big-Bang Nucleosynthesis and reionization). The
explosion of detailed observations in the last few decades has allowed for
precise measurements of the cosmological parameters within
Friedman-Lema\^itre-Robertson-Walker cosmologies leading to the $\Lambda$CDM
model: an apparently flat Universe, dominated by a cosmological constant, whose
matter component is dominantly dark. We describe and discuss the various
observational probes that led to this conclusion and conclude that the
$\Lambda$CDM model, although leaving a number of open questions concerning the
deep nature of the constituents of the Universe, provides the best theoretical
framework to explain the observations.
