Transient expression in tobacco cells is a convenient method for several purposes such as analysis of protein-protein interactions and the subcellular localization of plant proteins. A suspension of Agrobacterium tumefaciens cells carrying the plasmid of interest is injected into the intracellular space between leaf epidermal cells, which results in DNA transfer from the bacteria to the plant and expression of the corresponding proteins. By injecting mixes of Agrobacterium strains, this system offers the possibility to co-express a number of target proteins simultaneously, thus allowing for example protein-protein interaction studies. In this chapter, we describe the procedure to transiently express P-type ATPases in tobacco epidermal cells, with focus on subcellular localization of the protein complexes formed by P4-ATPases and their β-subunits.