I will briefly summarize forty years of search for eXtremely Metal-Deficient (XMD) galaxies and results of their studies, their current statistics and selection effects. Properties of several most extreme XMD galaxies will be described as well as the main motivation to study these very rare objects. I will review probable evolutionary path-ways of XMD galaxies and observational data indicating large diversity in their properties, consistent with different evolutionary scenarios. From the morphology and kinematics of neutral and ionized gas, the new evidence will be presented on the importance of induced SF in XMD galaxies. I will also address the issues of relatively small ages of the oldest unresolved stars in several most metal-poor galaxies, and finding of many XMD galaxies in void environment. The latter motivates the idea of unbiased study of evolution of void galaxies. Finally I will briefly describe the ongoing project to study dwarf evolution in a nearby Lynx-Cancer void and found to date several ’unevolved’ dwarf galaxies. While the work on statistis of evolutionary status differences is in progress, the high concentration of such unusual objects in this void hints on the sizable effect of very low-density environments on dwarf galaxy evolution.