Publisher Summary In analyzing the many and complex variables that affect the interaction between viruses and cells, present efforts can perhaps at best provide only model systems and working hypotheses that are not inconsistent with the available information. The study of virus-induced cell fusion is no exception. This chapter outlines the many areas in which further research is required. Many viruses are able to induce cell fusion. The increasing use in the past 5 years of virus-induced cell fusion as a research tool in cell biology, virology, and genetics is also a problem. Two distinct forms of virus-induced cell fusion are recognized. The first, so-called fusion from within is caused by viral products synthesized during intracellular replication of virus. The second type of fusion, so-called fusion from without, does not require intracellular virus replication and the synthesis of virus-coded products and is caused instead by components associated with the virus particle.