All cell types—bacteria, eukaryotes, and archaea—can be infected by particles called viruses. Viruses, the smallest known units of reproduction, are obligate intracellular parasites. Upon entering a cell, a viral genome subverts the cell’s machinery to reproduce more virus particles, usually killing the host cell. Alternatively, some viral genomes are copied into the host genome, where they replicate silently with the host. Degenerate viral genomes from ancient viral insertions take up a large part of the human genome. This chapter explores viral diversity, structure, life cycles, and ecology. Given the established and emerging viruses that threaten humans and agricultural crops, the relevance and importance of understanding viruses are obvious.