In order to perform a high number of assays on biological objects or chemical compounds which are sometimes impossible to obtain in large quantities, miniaturizing experiments has been the main issue. Standard technologies, such as microtiter plates have nearly reached the smallest sizes of samples they can handle. In order to miniaturize the assays, a further decrease in the size of the reactors in which reactions are performed is required. The use of small droplets provides new ways to miniaturize assays; moreover droplets are ideal for compartmentalizing biological objects or compounds. Droplets of equal sizes are produced in series and manipulated on demand providing quantitative and miniaturized versions of the microtiter plate assays and enables the parallelization of the assay.

In this chapter we will review droplet-based microfluidics: droplet production, flow, fusion, sorting, and detection. We will discuss how to design reactions in droplets and particularly how to maintain living cells in droplets and run biological reactions.