Introduction.

Abstract This chapter gives an overview on the current knowledge about endophytic fungi and bacteria, their diversity, their relationships with pests and pathogens, their distribution and activities inside the plant and their (potential) applications in developing more sustainable agricultural practices. Furthermore, the identification of chemical constituents synthesized by endophytes or by the endophyte-host plant association is discussed, as they can be most relevant for identifying novel compounds relevant for medicine, such as antibiotics and anticancer drugs, and for agriculture, such as biologically sound pesticides.