A new "uniplanar" circuit configuration for monolithic microwave integrated circuits (MMIC's) has been proposed. It uses a combination of coplanar waveguides and slotlines on one side of the substrate. The key components for the uniplanar structure are air bridges, which provide T junctions and transitions from coplanar waveguides to slotlines or vice versa. Novel hybrid circuits such as a magic T and a branch-line coupler have been fabricated and tested at K-band, and good performance has been achieved. This new circuit configuration is promising for applications in other microwave circuits.