The capacity to pattern biomolecules within microfluidic devices expands the scope of microfluidic technologies. In such patterned systems, surface-bound components remain localized, while the microfluidic network supplies reagents and removes waste products. This approach has enabled continuous protein expression from patterned DNA, chemical synthesis from immobilized enzymes, and cell capture assays. Here, we review methods to pattern surfaces within microfluidic devices. Patterns may be printed before or after the device is assembled; pre-bonding methods are compatible with well-established open-surface patterning protocols but present challenges for device bonding and alignment. Conversely, post-bonding methods are compatible with standard bonding procedures but rely on less established, sequential patterning protocols. Future progress will require consistent reporting of pattern signal and noise relative to controls.