Of the various minimal algebraic models of a simply connected space that have been constructed in the last decade, possibly the least understood and the one most suitable for application in geometry is K.-T. Chen's non-commutative algebra model. In this paper we give a complete exposition of Chen's methods and extend these in two directions: we establish a rational version of Chen's theory for simply connected semisimplicial complexes, and we show that the set of primitive elements of Chen's model is a Lie algebra model of the space whose generators correspond to cells in the space that represent non trivial rational homology classes.