This paper analyzes the process through which more than 650,000 off-grid households in rural Bangladesh decided to purchase a solar home system (SHS) from 1996 to 2010. The authors hypothesize that positive word of mouth is the primary driver of these sales. They tested the hypothesized diffusion process through a combination of semi-structured key informant interviews and an examination of 100 households in Panchua Village, including 60 with a SHS and 40 without a system.