An introduction is given to an algebraic formulation and generalisation of
the consistent histories approach to quantum theory. The main technical tool in
this theory is an orthoalgebra of history propositions that serves as a
generalised temporal analogue of the lattice of propositions of standard
quantum logic. Particular emphasis is placed on those cases in which the
history propositions can be represented by projection operators in a Hilbert
space, and on the associated concept of a `history group'.Comment: 14 pages LaTeX; Writeup of lecture given at conference ``Theories of
fundamental interactions'', Maynooth Eire 24--26 May 1995
