Egg-related coelomitis describes an acute or chronic, usually diffuse, coelomitis involving egg yolk, egg albumin, and/or eggshell occurring with or without bacterial infection (Fig. 1A and B). The older term “egg yolk peritonitis” has fallen out of favor as birds have a coelom rather than a peritoneum, and the offending substance can be not just the yolk of the egg but other parts of the egg. It is common for chickens to have some degree of egg-related coelomitis, and mild cases are commonly encountered at necropsy in production hens. Generally, chickens tolerate mild peritonitis better than parrots. Causes usually involve retrograde movement of shelled or shell-less eggs from the oviduct back into the coelomic cavity because of oviductal bacterial infection, oviductal impaction, or abnormal confirmation of the oviduct. Heavy-production hens or those with inadequate calcium in their diet can have calcium depletion and uterine inertia leading to retrograde flow of egg material. Bacterial infections commonly involve Escherichia colimigrating up the oviduct from the vent.