Reaction injection molding using in situ photoinduced polymer macromer gelation in microfluidic channels was applied to the fabrication of poly(ethylene glycol) (PEG) hydrogel microstructures. These hydrogel microstructures were fabricated using poly(dimethylsiloxane) (PDMS) microchannels as mold inserts alone or in combination with photolithography. These microstructures were formed by flowing a gel precursor solution through the microfluidic network, exposing to light, and finally removing the PDMS mold. Microchannels as narrow as 10 mum wide could be used for molding PEG hydrogels, and the resulting three-dimensional hydrogel microstructures did not delaminate from substrates treated with 3-(trichlorosilyl)propyl methacrylate, a gel adhesion promoter. By exploitation of the laminar flow and poor mixing conditions in a microfluidic channel, single microstructures with heterogeneous chemistries were also created, using peptide-modified structures to promote cell adhesion as a model.