  To appear in proceedings of II Workshop on ``Constraints Theory and
Quantisation Methods''Montepulciano (Siena) 1993}
  General discussion of the constraints of 2+1 gravity, with emphasis on two
approaches, namely the second order and first order formalisms, and comparison
with the four dimensional theory wherever possible. Introduction to an operator
algebra approach that has been developed in the last few years in collaboration
with T.Regge, and discussion of the quantisation of the g=1 case.
