Cleantech entrepreneurship is major area of sustainable entrepreneurship that has global potential, yet extant studies in the entrepreneurship or international business domains have not managed to provide any explanations on how enterprises across a new arising sustainability-related industry sector such as the cleantech sector grow, succeed and internationalize. This chapter responds to those omissions in knowledge, by discussing the rise of cleantech entrepreneurship, specifically by illustrating it through the theoretical lens of population ecology theory. In doing so, this chapter embraces the population level of analysis on the emergence, development and entrepreneurial internationalization of enterprises in the context of cleantech entrepreneurship, using the empirical context of Finland as an example. We illustrate how the new breed of cleantech firms, oftentimes small specializing start-ups operating in a narrow niche, seeks to outcompete traditional technology firms which are usually generalist multinationals with diversified markets and technologies at hand. In this study, cleantech  sector firms are highlighted as a distinct population of organizations with the emergence, development and internationalization of this population are examined from the perspective of organizational ecology, thus introducing  a new approach to explain how entrepreneurship in new types of sectors  such as the cleantech sector arises and functions.