Cells arise from cell divisions of parent cells. During sexual reproduction, a new organism develops from one single cell, the fertilized egg. Cell division occurs in cycles where the DNA is doubled in S phase by DNA replication and then partitioned onto the two daughter cells during mitosis. In this chapter, we discuss the mechanisms of mitosis and cell cycle control in detail. Moreover, we look at the signal transduction pathway by which growth factors regulate cell proliferation and differentiation. In order to maintain homeostasis of all cell types in a multicellular organism, also cell death (apoptosis) is regulated. A cellular suicide program is initiated by cells when they either are damaged or when they have fulfilled their functions and are not needed anymore. This program is initiated by extracellular signals or by intracellular mechanisms, which often involve the release of mitochondrial proapoptotic factors. These signals activate caspases, which then orchestrate the demise of the cell.Finally, we discuss the structure and function of viruses. Viruses carry genetic information as RNA or DNA within an envelope or a capsule. They have to enter a host cell in order to replicate because they do not have ribosomes and thus cannot manufacture the proteins which are encoded in their nucleic acids.