Paper is a favorable material for microfluidic devices owing to its ability to facilitate spontaneous transportation of solutions via capillary action, thus eliminating the need for external instruments. In addition, paper is an inexpensive and lightweight material that can be easily discarded after use, easily folded, and fabricated via simple and cost-effective techniques. Test strips are widely used paper-based devices. However, sensing performance and solution processing functions achieved by test strips are limited. To address this issue, highly sophisticated paper-based devices have been realized. Even if we limit our discussion to devices based solely on electrochemical sensing principles—excluding other types such as optical sensors—numerous paper-based devices have been developed, each offering diverse functionalities for detecting a wide range of analytes. To fabricate such devices, electrodes are typically formed via thick-film techniques such as screen printing. Solution reservoirs, flow channels, and reaction chambers are separated from other parts by making the other parts hydrophobic and water-impermeable via techniques such as wax printing. To improve the functionality of two-dimensional devices fabricated using a single sheet of paper, various three-dimensional structures have been realized by stacking paper layers. Necessary reagents are stored at designated parts in dried forms. Moreover, various techniques have been used for solution control; these include timed sequential delivery of multiple solutions using special flow-channel geometries, utilization of valves that leverage the property of paper, wettability control using flow channels of varying hydrophobicity, and solution exchange by folding or sliding paper.