Density evolution is a dynamic system in a space of probability
distributions representing the progress of iterative decoders in
the infinite block length limit. In this paper we establish some
basic results concering this process. In particular we show that
the decoding threshold is equivalent to to appearance of non-trivial
fixed point solutions to the density evolution equations. In the
case of LDPC codes we prove the sufficiency of the previously
published stability condition for stability of the DELTAINFINITY
fixed point and slightly strengthen the necessity result.