Plant pests and diseases, whether introduced from overseas (exotics) or endemics that have extended beyond their native range, represent a major threat to Australia’s agriculture, forestry and natural environments. All plants are at risk of being damaged by unwanted pests and diseases. Plant pests include animals such as herbivorous insects, birds and mammals that cause significant damage to the plants while feeding on them. Plant diseases are caused by viruses, bacteria or fungi that can reduce plant productivity or even kill them. Some plant diseases are vectored by insects. Both pests and diseases can be either exotic introductions into Australia or endemic species that either attack introduced plant species or have moved out of their natural geographic range. This chapter will focus on insect plant pests and viral, bacterial or fungal plant diseases within peri-urban environments. It does not discuss pest plants (weeds) as they have been discussed in the previous chapter.