The idea of thin films is simple: use mostly low cost material (glass, metal, plastic) and very little high cost semiconductor. A micron or so of semiconductor is about 2-6 g/m2; even ultraexpensive material (say, $1000/kg) only costs pennies per watt at this level. This idea has been around as long as PV, but the difficulty has been developing semiconductors that would work well enough (have high enough conversion efficiency) and then finding ways to actually make them cheaply at high yield. And thin films have their own peculiar stability issues, both intrinsically and at the module level, which have also added to the challenge. However, throughout, the idea of thin films has maintained its power, so that no single failure or long, costly delay has destroyed thin film development. But setbacks have taught us something about what is needed. Fortunately, thin films today are approaching technical readiness (Zweibel et al., 2004), achieving measurable and growing market share, and may finally show that they have an excellent chance of reaching truly low cost. This study makes an attempt to explain why thin films are likely to be successful for meeting large scale PV electricity demands.