We provide a simple, unified proof of Birkhoff's theorem for the vacuum and cosmological constant case, emphasizing its local nature. We discuss its implications for the maximal analytic extensions of Schwarzschild, Schwarzschild(-anti)-de Sitter and Nariai spacetimes. In particular, we note that the maximal analytic extensions of extremal and over-extremal Schwarzschild-de Sitter spacetimes exhibit no static region. Hence the common belief that Birkhoff's theorem implies staticity is false for the case of positive cosmological constant. Instead, the correct point of view is that generalized Birkhoff's theorems are local uniqueness theorems whose corollary is that locally spherically symmetric solutions of Einstein's equations exhibit an additional local Killing vector field.PACS numbers: 04.20.Cv, 04.20.Gz Birkhoff's theorem, that any spherically symmetric vacuum solution of Einstein's equations is locally isometric to a region in Schwarzschild spacetime, is not only a classic contribution to general relativity but is also an important tool in gravitational physics and cosmology. First proven for the vacuum Einstein equations, this theorem is traditionally credited to Birkhoff [1]; however it was recently rediscovered by Deser, when providing an alternate proof, that this theorem was published two years earlier by Jebsen [2,3,4]. (An english translation of Jebsen's paper with an introduction by Deser was published in General Relativity and Gravitation in 2005 [5, 6].) In older references, this theorem is credited as independently discovered by not only Birkhoff and Jebsen but also Alexandrow [7] and Eiesland [8].1 The multiple provenance of this theorem is a testament to its significance in general relativity. It is natural to consider its extension to other situations in gravitational physics. Birkhoff's theorem readily generalizes to the inclusion of electromagnetic fields [10], cosmological constant [8], with explicit exhibition of both the Schwarzschild-de Sitter and Nariai solutions in [11,12] (also in [13] without citation of previous references), and to other symmetry groups [14,15].2 Generalized Birkhoff's theorems have also been proven in lower and higher dimensions [16,17,18,19,20,21], certain alternate theories of gravity [22,23,24,25,26,27,28] including Lovelock gravity [29,30,31,32], and shown not to hold on Randall-Sundrum branes [33] and in some modified theories of gravity [34,35].Although Birkhoff's theorem is a classic result, many current textbooks and review articles on general relativity no longer provide a proof or even a careful statement of the theorem. Frequently it is cited as proving that the spherically symmetric vacuum solution is static. This is clearly not the case as recognized in many (but not all) proofs of the theorem (See, for example, the comment on Jebsen's proof in [36]). The interior region of the maximal analytic extension of Schwarzschild spacetime is not static; instead, it exhibits a spacelike Killing vector field. Of course, other regions of this spacetime do exhi...