In these lectures I make an introduction to chiral unitary theory applied to the meson-baryon interaction and show how several well-known resonances are dynamically generated, and others are predicted. Two very recent experiments are analyzed, one of them showing the existence of two Λ(1405) states and the other one providing support for the Λ(1520) resonance as a quasi-bound state of Σ(1385)π. The use of chiral Lagrangians to account for the hadronic interaction at the elementary level introduces a new approach to deal with the modification of meson and baryon properties in a nuclear medium. Examples of it for $$\bar K$$ andø modification in the nuclear medium are presented