Interest in investing in certain themes has grown with the proliferation of exchange-traded funds (“ETFs”). Indeed, creators of ETFs have launched and marketed theme-oriented ETFs in anticipation of potential investor demand. Climate change is a broad investment theme that has spawned clean energy ETFs and an array of sub-theme ETFs including solar, water and batteries. Understanding how a clean energy ETF invests and the resultant outcomes are important when making such a theme investment. Important as well is knowing how such a theme investment interacts with other portfolio holdings. A review of clean energy experience may be informative for assessing the general role of theme investing.