In this appendix I sketch the main lines of development of the research in quantum gravity, from the first explorations in the early 1930s (the thirties) to nowadays.

I have no ambition of presenting complete references to all the important works on the subject; some of the references are to original works, others to reviews where references can be found. Errors and omissions are unfortunately unavoidable and I apologize for these. I have made my best efforts to be balanced, but in a field that has not yet succeeded in finding consensus, my perspective is obviously subjective. Trying to write history in the middle of the developments is hard. Time will pass, dust will settle, and it will slowly become clear if we are right, if some of us are right, or – a possibility never to disregard – if we all are wrong.

I am very much indebted to the many friends that have contributed to this historical perspective. I am particularly grateful to John Stachel, Augusto Sagnotti, Gary Horowitz, Ludwig Faddeev, Alejandro Corichi, Jorge Pullin, Lee Smolin, Joy Christian, Bryce DeWitt, Cecile DeWitt, Giovanni Amelino-Camelia, Daniel Grumiller, Nikolaos Mavromatos, Stanley Deser, Ted Newman and Gennady Gorelik.