  We present an overview of the advances in the physics of polaritons in
semiconductor microcavities, starting from their first discovery in 1992. After
summarizing the research during the early years, when the basic optical
properties were investigated, we describe the important results related to the
phenomenon of parametric polariton scattering, highlighting its link to
non-classical polariton physics. We conclude with a perspective view of the
future research in the domain, focusing on the exciting developments in the
direction of polariton quantum collective phenomena and quantum nanodevices
