Plant hormones (phytohormones) are signal molecules produced within the plant, and occur in very low concentrations. In the present chapter, the current knowledge on the regulation of biotic and biotic stress responses by plant hormones is summarized with special focus on the novel insights into the complex hormonal crosstalk of classical growth stimulating plant hormones within the naturally occurring biotic and abiotic multistress environment of higher plants. The MAPK- and phytohormone-cascades which comprise a multitude of single molecules on different signalling levels, as well as interactions and cross-regulations within and between these signalling pathways allow very specific and fine-tuned modulation of plant immunity. The endoplasmic reticulum (ER)-associated protein degradation system (ERAD) is a quality control system that degrades improperly folded proteins from the secretory pathways through ubiquitination. The wide range of biotic and abiotic stresses that affect crop plants limits agricultural production.