  The topic of the glass transition gives rise to a a wide diversity of views.
It is, accordingly, characterized by a lack of agreement on which would be the
most profitable theoretical perspective. In this chapter, I provide some
elements that can help sorting out the many theoretical approaches,
understanding their foundations, as well as discussing their validity and
mutual compatibility. Along the way, I describe the progress made in the last
twenty years, including new insights concerning the spatial heterogeneity of
the dynamics and the characteristic length scales associated with the glass
transition. An emphasis is put on those theories that associate glass formation
with growing collective behavior and emerging universality.
