  We summarize the progress in identifying and observational study of extremely
metal-deficient (XMD) gas-rich galaxies (BCGs, dIr and LSBDs). Due to volume
limitations only following issues are addressed: sample creation, some
statistical data, Colour-Magnitude Diagrams (CMD) and ages, the case of SBS
0335--052 system, and probable evolution paths of various XMD objects.
