This thesis at first introduces planar transformers and their applications to RF circuits. A method is given to validate the impedance transformation and calculate the insertion loss for a transformer with unequal turn windings. A physical lumped-element model has been established for a planar transformer-based balun implemented using integrated passive device technology. This thesis next presents CMOS power combiner designs with two differential ports and one single-ended port. Such designs equivalently integrate two baluns and one single-ended power combiner into a single circuit and therefore have the advantages of saving chip area and reducing insertion loss. In addition, the designs consider an annular ground structure to achieve lower insertion loss or better balance property. Finally, this thesis presents a balun using a stacked coil transformer in an organic build-up substrate. For this purpose, a complete design flow and an impedance matching technique have been given.