  CODEX and ESPRESSO are concepts for ultra-stable, high-resolution
spectrographs at the E-ELT and VLT, respectively. Both instruments are well
motivated by distinct sets of science drivers. However, ESPRESSO will also be a
stepping stone towards CODEX both in a scientific as well as in a technical
sense. Here we discuss this role of ESPRESSO with respect to one of the most
exciting CODEX science cases, i.e. the dynamical determination of the cosmic
expansion history.
