The world is now facing the reality that fossil fuels are a finite resource that will be exhausted someday, that global consumption is outpacing the discovery and exploitation of new reserves, and that the global environment is worsening due to increasing greenhouse gas (GHG) emissions caused by traditional fossil fuels. As a result, there is a renewed push for alternative energy technologies, as well as technologies that can enhance recovery, transportation, and energy utilization and conversion efficiencies. In this entry, a review of current alternative energy technologies and their relevance in various energy sectors will be offered, including their most recent progress and the remaining challenges to overcome. Technology barriers and research/development opportunities for further growth in each category are outlined, and future projected growth is discussed in brief.