Since its inception in 2001, Heegaard Floer homology has developed into such a large area of low-dimensional topology that it has become impossible to overview all of its applications and ramifications in a single paper. For the state of affairs in 2004, see the excellent survey article of Ozsváth and Szabó [70]. A decade later, this work has two goals. The first is to provide a conceptual introduction to the theory for graduate students and interested researchers, the second is to survey the current state of the field, without aiming for completeness.After reviewing the structure of Heegaard Floer homology, treating it as a black box, we list some of its most important applications. Many of these are purely topological results, not referring to Heegaard Floer homology itself. Then, we briefly outline the construction of Lagrangian intersection Floer homology, as defined by Fukaya, Oh, Ono, and Ohta [16]. Given a strongly s-admissible based Heegaard diagram pΣ, α, β, zq of the Spin c 3-manifold pY, sq, we construct the Heegaard Floer chain complex CF 8 pΣ, α, β, z, sq as a special case of the above, and try to motivate the role of the various seemingly ad hoc features such as admissibility, the choice of basepoint, and Spin c -structures. We also discuss the proof of invariance of the homology HF 8 pΣ, α, β, sq up to isomorphism under all the choices made, and how to define HF 8 pY, sq using this in a functorial way (naturality). Next, we explain why Heegaard Floer homology is computable, and how it lends itself to the various combinatorial descriptions that most students encounter first during their studies. The last chapter gives an overview of the definition and applications of sutured Floer homology, which includes sketches of some of the key proofs.Throughout, we have tried to collect some of the important open conjectures in the area. For example, a positive answer to two of these would give a new proof of the Poincaré conjecture.Acknowledgement. I would like to thank for their comments on earlier versions of this paper. ι ÝÑ HF 8 pY, sq π ÝÑ HF`pY, sq δ ÝÑ . . .
This gives rise to the invariantHFr ed pY, sq " cokerpπqkerpιq " HFŕ ed pY, sq, where the isomorphism is given by the coboundary map δ.