  The field of ultrarelativistic heavy ion collisions is today a flourishing
activity both on the experimental and on the theoretical side. Although the
theoretical justifications to study these collisions was given already more
than three decades ago and the experimental studies have a history of more than
25 years we are still very much in the dark as to the details of the processes
and of the characteristics of the matter created in collisions. Increasing the
energy of collisions has brought new insights but has also resulted with new
challenges. In the present paper I will try from a personal perspective to
report on the answers we have collected and on the problems we are faced with.
The account is partial, taking into account that it is impossible to render
justice to every aspect of the field.
