Current Uncertainties in the Use of Cepheids as Distance Indicators

The methods of calibrating the luminosities of galactic Cepheids and determining Cepheid reddenings are considered in some detail. Together with work on NGC4258 this suggests that the calibration presented is valid to about 0.1mag (s.e.) at least for Cepheids with near-solar abundances. Metallicity effects are considered, partly through the use of non-Cepheid moduli of the LMC. To reduce the uncertainty substantially below 0.1mag will require extensive work on metallicity effects. Non-linearities in the period-luminosity and period-colour relations will also need to be considered as will the need to distinguish unambiguously between fundamental and overtone pulsators.