Bias lines are a necessary part of every active circuit, but there is little information on their design in textbooks or publications. In this article, a study on bias lines for millimeter wave gallium nitride (GaN) millimeter-wave monolithic microwave integrated circuits is performed with a focus on integration of bias lines with matching and stability networks. Several bias line structures are fabricated separately in GaN and tested, comparing different techniques. The study is extended to practical design for a single-stage amplifier.