Publisher Summary Inclusion bodies can be simply defined as intracellular structures produced de novo as a result of viral infections. They may contain virus particles, virus-related materials, or ordinary cell constituents in a normal or degenerating condition, either single or, more often, in various proportions. By this definition, most, if not all, plant viruses are liable to induce formation of inclusion bodies of some sort. These may be crystalline or amorphous (or both) in the same cell, and may vary in size. Thus, some inclusions are readily seen with the light microscope, as they constitute bulky masses often attaining 10-20 μm along the major axis, whereas others are only visible with the electron microscope. This is a broad interpretation of the concept of inclusion body. Intracellular inclusions arise as a consequence of disturbed cell metabolism following virus infection, but are not necessarily a by-product of this process. In some instances, inclusions constitute a more or less inert accumulation of the virus particles or excess viral material, but there is mounting evidence that, in many cases, inclusion bodies play a primary role in the synthesis and/or assembly of viral components. Therefore, knowledge of their fine structure and composition seems fundamental to an understanding of their probable function. In this chapter, rather than reviewing plant virus inclusions comprehensively, it discusses the most significant findings in this field in the context, whenever possible, of an interpretation of the role of inclusions in the infection process.