Single-domain antibodies fold into their native and functional structure once secreted at the surface of bacterial or yeast cells and single-domain antibody display have been used for both screening purposes and for preparing whole-cell biosensors. Single-domain antibody displaying bacteria can co-express cytoplasmic fluorescent proteins for obtaining colored living immunoreagents suitable for light microscopy and flow cytometry applications. Yeast surface display has been largely used also for panning antibody fragment libraries because enables the sorting of all the selected clones by flow-cytometry. This represents a huge advantage over phage display that requires manual screening of the clones recovered after panning.