After thousands of years, the traditional use of RES is changing to meet modern needs. The book presents major developments with modern technologies and applications of RES for over fifty years. It describes an entire creative era, with Greece offering new ideas and contributing to the development of technology by participating ambitiously in the European and international scene for the great penetration of wind and solar energy into electricity. It includes research projects and interesting missions with comments on RES technologies and international developments that marked their dynamic progress and development in the years that followed. Special emphasis is placed on energy transition, the greatest challenge of the 21st century, which will also bring transformation to the economy and society. RES, mainly solar and wind energy with low production costs, along with storage and advanced management and control strategies, will be the main sources with environmental and social benefits, without the high external costs in health and the environment. As described, the expansion of electricity use becomes attractive, while hydrogen production for all other energy uses begins. A 100% RES electric system is feasible and competitive with fossil fuels prevailing today.