This paper investigates how the structure of cooperation in an industry influences the dynamics of entry by start-up firms. Competition over technological dominance induces the entry of start-up firms into new subfields as incumbent firms seek to expand the consumer base using their technology. By cooperating, incumbent firms succeed to varying degrees in establishing their technologies as a dominant standard by building central positions in a cooperative network. Start-ups tend to enter, however, if there is reasonable certainty that a dominant technology has been established. We find strong support for the relationship between network centrality, as a measure of technological dominance of a standard, and the entry of start-up firms into the semiconductor industry.