A review of observational evidence in favour of a metallicity dependence of WN and WC stars is presented .
New near-IR studies of Milky Way , LMC and SMC early-type WN stars are presented , with weaker winds amongst WN stars containing hydrogen .
A metallicity dependence is supported for WN stars with hydrogen , with dM / dt \propto Z ^ { \alpha } with \alpha \sim 0.8 \pm 0.2 The influence of CNO content upon WN subtypes is discussed .
Earlier WN spectral types are expected ( and observed ) at lower metallicity due to the abundance sensitivity of N iii-iv classification diagnostics .
Recent physical and chemical results of WC stars in the Milky Way and LMC are discussed , suggesting a metallicity dependence of \alpha \sim 0.6 \pm 0.1 .
Earlier WC spectral types are predicted ( and observed ) in lower metallicity galaxies , due to the dependence of the C iii classification diagnostic on wind density .
WO stars reveal lower wind velocities at lower metallicity , whilst the situation for WN and WC stars is unclear .
Finally , the influence of a WR metallicity dependence upon the ionizing flux distributions and optical line luminosities is addressed , with particular regard to I Zw 18 .
Weaker winds at low metallicity would imply harder ionizing flux distributions and lower line luminosities , arguing for an substantially increased number of WR stars with respect to standard calibrations , exacerbating difficulties with single star evolutionary models at very low metallicity .