Incorporating energy from renewable resources into power system planning is an important issue. As countries generate more energy from renewable sources, that energy must become part of the power-system planning process. In 2004, investments in all forms of clean energy represented about 20 percent of total global investment in generation capacity. By 2011, that share had topped 40 percent. This paper explains the various reasons behind the push for renewable sources, chief among them being reducing climate-altering emissions. The paper answers the following questions giving a brief synopsis of each: What is the key challenge? What solutions are available? What are the key lessons learned. A few case studies are also used throughout to illustrate key points.