Search for and study of extremely metal-deficient galaxies

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.