Using a characterization of parabolics in reductive Lie groups due to Furstenberg, elementary properties of buildings, and some algebraic topology, we give a new proof of Tits' classification of 2-transitive Lie groups.Among many other results, Tits classified in [41] all 2-transitive Lie groups. His proof is based on Dynkin's classification of maximal complex subalgebras in complex simple Lie algebras; it is long and depends on consideration of various cases. Since the resulting list of groups is also long (at least in the affine case), it is clear that there cannot be a very short proof of the full classification. On the other hand, Lie theory has developed since the time [41] was written. In particular, Tits himself changed the picture through his theory of buildings (as he pointed out, his paper [41] was one of the motivations for him to invent buildings). The language, the methods, and the terminology have changed since then, and it is natural to look for a new (and shorter) proof of Tits' classification. Note also that the proof presented in [41] IV F 1.2, p. 222, does not cover certain real forms of exceptional groups -a footnote on p. 223 asserts that Tits found a proof for these cases, too, after the manuscript went into print; see also loc.cit. p. 240. The details were never published.Almost at the same time as Tits, Borel [4] determined all 1-connected spaces X which admit a 2-or 3-transitive Lie group action; however, Borel did not classify the corresponding groups. His proof relies on spectral sequences, Freudenthal's theory of ends, and on the results of Borel and De Siebenthal about homogeneous spaces of positive Euler characteristic.In this paper, we give a complete proof for Tits' classification. The main ingredients are a characterization of parabolics in Lie groups due to Furstenberg, elementary properties of buildings, some algebraic topology (certainly more elementary than the machinery employed in Borel's work [4]), and representation theory of semisimple (compact) Lie groups. * Supported by a Heisenberg fellowship by the Deutsche Forschungsgemeinschaft 1 As we remarked before, the only published proof for the classification is [41]. The classification in the affine case is also stated (but not proved) in Völklein [47], and some remarks on the strategy of Tits' original proof can be found in Salzmann et al. [36] 96.15 and 96.16.Related results for other classes of groups are Knop's classification [26] of 2-transitive actions of algebraic groups over algebraically closed fields in arbitrary characteristic (which is achieved by quite different methods), and the classification of all finite 2-transitive groups; see Dixon-Mortimer [11] 7.7 for a description of these groups. In the course of our classification, we recover Knop's result for the special case of complex algebraic groups.The main results of the classification are as follows.Theorem A Let G be a locally compact, σ-compact topological transformation group acting effectively and 2-transitively on a space X which is not totally...