Multiplicity of Massive Stars - a Review Hans Zinnecker (Astrophys. Inst. Potsdam) Multiplicity of massive stars is prevalent and interesting to study, as it may hide important clues to understand the formation of massive stars in multiple systems (Zinnecker 2003, IAU-Symp. 212, p. 80; Zinnecker and Bate 2002, ASP-Conf. 267, p. 209; Bonnell and Bate 2005, astro-ph/0506689). First, the multiplicity of the OB stars in the Orion Trapezium Cluster as well as the Sco-Cen-Lup association will be reviewed, based on previous spectroscopic, speckle, and adaptive optics surveys. Are there any major systematic differences in multiplicity between dense clusters and loose associations? Multiplicity results in other young open clusters, both rich and poor in massive stars, will be rediscussed with an eye on a physical interpretation (e.g. NGC 6231, M16, Tr 14, Tr 16; NGC 2264, NGC 2362; cf. Mermilliod and Garcia 2001, IAU-Symp. 200, p. 191). The multiplicity of runaway OB stars (Mason et al. 1998) will also be revisited. They tend to be single objects, ejected from a cluster after a dynamical interaction. Further, our current knowledge about the close binary nature among massive stars in the starburst clusters R136 and NGC 3603 will briefly be summarized (Bosch et al. 2001, Massey et al. 2002; Hofmann and Weigelt 1986, Stolte et al. 2005). We must distinguish between hard and soft binaries. Bonnell and Bate's (2005) idea for origin of the hard tight massive binaries will discussed in terms of an accretion scenario. We present our own new idea for the origin of the Orion and other Trapezium-like systems (e.g. W3-IRS5) due to the merging of sub-clusters, each sub-cluster contributing its own central massive star (itself likely to be multiple). This idea is based on the numerical simulations of hierarchical cluster formation by Bonnell, Bate, and Vine (2003). PS: Orbital solutions for some massive binaries are known or under way (theta-1 Ori, 15 Mon, HD 93129A, WR20a). OGLE III will discover many massive eclipsing binaries in the Magellanic Clouds, several are known already.