We give an elementary introduction to hyperkähler manifolds, survey some of their interesting properties and some open problems. Contents 1. C ∞ manifolds 3 2. Riemannian manifolds 7 3. Kähler manifolds 11 4. Holomorphic symplectic manifolds 14 5. Moduli of hyperkählers, the Beauville-Bogomolov form, the period domain and the period map 20 6. Twistor spaces and twistor conics 27 7. Examples of hyperkählers in dimension 2 and beyond, by Samir Canning 29 8. Basic properties of Lagrangian fibrations of Hyperkählers, by Yajnaseni Dutta 32 9. Rational curves on K3 surfaces and Euler characteristics of Moduli spaces, by David Stapleton 35 References 37