We show that the progress of technological knowledge is an inherently ecological process, wherein the growth race of each technology domain depends on dynamics occurring in other technology domains. We identify two sources of ecological interdependence among technology domains. First, there are symbiotic interdependencies, implying that the race of growth of one technology domain is driven by the advances made in other technology domains. Second, some technology domains compete with each other, implying that the race at which a given technology domain advances varies inversely with the competitive pressure it receives from other technology domains. Based on all the technological knowledge patented in the United States between 1975 and 1999, we find statistical support for our argument and hypotheses.The question of why some technology domains grow faster than others has drawn the attention of both sociologists and economists (Carnabuci 2006). As a result, several factors have been identified that bear on this question. Some scholars have argued that the uneven progress of technological knowledge reflects the changing needs of the marker (Schmookler 1966), while others have emphasized the importance of institutional regimes (Bonaccorsi and Thoma 2007). Also, it has been argued that the growth of technological knowledge depends on "supply-push" factors, such as the ease and precision with which experimental tests can be carried out in a given technology domain (Nelson 2003), its inherent technical bounds (Girifalco 1991), and its degree of specialization (Carnabuci and Bruggeman 2009). Furthermore, the progress of technology domains has been argued to hinge on the social construction of what becomes legitimately accepted as "new" knowledge by the relevant epistemic community (Latour 1987;Knorr-Cetina 1999).An implicit assumption common to all these explanations is that technology domains grow independently of each other. Departing from this assumption, our main contention in this paper is that technology domains evolve in an ecologically interdependent fashion. We reckon that two kinds of ecological pressure impact the growth of technology domains. First, there may be symbiosis among some technology domains. That is, the race of growth of one technology domain may be accelerated by the advances made in other technology domains. Second, some technology domains may be in competition with each other. When thisThe author thanks Jeroen P. Bruggeman for his cooperation in an earlier version of this article, Afonso Gambardella for his useful insights and support, and the editor and anonymous reviewers for their constructive criticisms and suggestions.