Apparatus and methods are provided for constructing waveguide-to-transmission line transitions that provide broadband, high performance coupling of power at microwave and millimeter wave frequencies. More specifically, exemplary embodiments of the invention include wideband, low-loss and compact coplanar waveguide-to-rectangular waveguide transition structures and asymmetric coplanar stripline (or coplanar stripline)-to-rectangular waveguide transition structures that are particularly suitable for microwave and millimeter wave applications.