This article describes the design, function, and application of simple microfluidic networks as conduits for the patterned delivery of chemical reactants onto a substrate. It demonstrates how such networks, made in an elastomer, allow simultaneous delivery of functionally distinct molecules onto targeted regions of a surface (Delamarche, E. et al. Science 1997, 276, 779-781). Microfluidic networks generally consume less than microliter quantities of solution and are thus well suited for use when the required reagents are scarce or precious, as often occurs in experiments and technologies that place biochemicals on solid planar substrates. We illustrate some of the particular challenges of doing chemistry inside the narrow confines of capillaries defined by fluidic networks, in addition to the advantages attendant to this approach, in the context of forming patterned arrays of different, and functional, immunoglobulins useful in highly localized biological assays.