Generalized Kähler geometry is the natural analogue of Kähler geometry, in the context of generalized complex geometry. Just as we may require a complex structure to be compatible with a Riemannian metric in a way which gives rise to a symplectic form, we may require a generalized complex structure to be compatible with a metric so that it defines a second generalized complex structure. We explore the fundamental aspects of this geometry, including its equivalence with the bi-Hermitian geometry on the target of a 2-dimensional sigma model with (2, 2) supersymmetry, as well as the relation to holomorphic Dirac geometry and the resulting derived deformation theory. We also explore the analogy between pre-quantum line bundles and gerbes in the context of generalized Kähler geometry.A striking feature of the Courant bracket is that it has symmetries fixing the underlying space M. Any closed 2-form B ∈ Ω 2,cl (M) acts on TM, preserving the Courant bracket, via the bundle map:(1.2)Because of this symmetry, we may "twist" or modify the global structure of TM as an orthogonal bundle with a Courant bracket, keeping its local structure unchanged. These twisted structures are therefore classified by a characteristic class in H 1 (Ω 2,cl (M)) and are called exact Courant algebroids [13,18,19].