This FULL COLOR publication focuses on the relatively small number of microorganisms (primarily fungi) and arthropods (primarily insects) that are capable of causing noticeable damage to oaks in California. We have included agents that cause serious damage to oaks, as well as some common agents that produce conspicuous impacts even if they are not especially detrimental to oak health. Conspicuous but relatively inconsequential agents, such as cynipid gall wasps, often attract more attention than more cryptic agents that can severely impact oak health and structural integrity, such as canker...