Abstract. We study the external and internal Zappa-Szép product of topological groupoids. We show that under natural continuity assumptions the Zappa-Szép product groupoid isétale if and only if the individual groupoids areétale. In our main result we show that the C * -algebra of a locally compact Hausdorffétale Zappa-Szép product groupoid is a C * -blend, in the sense of Exel, of the individual groupoid C * -algebras. We finish with some examples, including groupoids built from * -commuting endomorphisms, and skew product groupoids.