The organisms described in this chapter are eukaryotic and they belong to two different kingdoms, Chromista and Protozoa. They are often incorrectly referred to as 'fungi' because they have some morphological traits in common with true fungi. Most of them are microscopic, but some can make structures visible to the naked eye. The major group of plant pathogens described in this chapter is the oomycetes, which belong to kingdom Chromista. An overview of the characteristics and classification of the kingdoms are presented, and species causing important plant diseases are described.