The current interest in green technologies has directed attention to the use of plant systems for several applications, including traditional crop plant systems used for biomass production, large-scale synthesis of a great number of recombinant proteins, and biofuels production. In this context, plant viruses are very useful instruments for plant biotechnology applications, constituting suitable tools for heterologous gene expression. Virions are particles with a complex composition, but their stability allows them to be used for the development of numerous biotechnological applications and as research tools for plant functional genomics studies. The development of infectious full-length viral clones is a strategy extensively employed as an alternative tool for introducing viruses into plants via inoculation with Agrobacterium tumefaciens. Another strategy, called RNA interference, a plant gene expression regulation mechanism based on post-transcriptional gene silencing, has extensively been employed to down-regulate the expression of endogenous transcripts and displays a number of biotechnological applications. Additionally, transgenic expression of viral proteins has been used to achieve pathogen-derived resistance, a mechanism that confers resistance to viral infections in agricultural crops. In this chapter we will discuss several strategies and methods for plant gene expression which employ plant viruses developed over the past decade.