While startups are acknowledged for their potential to address sustainability issues, little is known on how to assess their impact, given the uncertainty they deal with and their lack of resources. This paper investigates the ones that are supposed to be ‘best-in-class’ in that matter, that is, startups targeting sustainability, in order to explore how they integrate sustainability impact assessment in their entrepreneurial process. We conducted a multiple case study of eight sustainable startups, based on a 2-year longitudinal research in their incubator to gather multiple sources of information. Our results revealed that the integration of the triple bottom line in the entrepreneurial process has a major effect on startups’ sustainability impact assessment practices. ‘Born-sustainable startups’, which have aimed for the triple bottom line since idea generation, have more robust tools and routines than ‘Transitioned sustainable startups’, which integrated the triple bottom line during prototype/validation.
