The endoplasmic reticulum (ER) is a specialized organelle that connects almost all subcellular structures from the plasma membrane to the nucleus. The ER is involved in secretory protein synthesis, folding, and processing. Evidence has emerged that the ER is at the frontier of the battle between plant hosts and pathogens. Its structural and functional homeostasis is crucial for the survival of plant cells. Pathogens secrete effectors to take over normal functions of the ER, while host plants fight back to activate ER stress responses. Exciting advances have been made in studies on host plant–pathogen dynamics during the past decades, namely some new players involved have been recently resolved from both pathogens and hosts. In this review, we summarize advances in identifying structural characteristics of the key pathways and effectors targeting the ER. Newly identified ER-phagy receptors and components downstream of inositol-requiring 1 (IRE1) will be described. Future studies will be envisaged to further our understanding of the missing parts in this dynamic frontier.