This thesis is focused on the modeling and characterization of planar interconnects operating at microwave frequencies covering both, integrated circuit (IC) and printed circuit board (PCB) technologies. In particular, the state-of-the-art of the field is advanced by providing physically-based models incorporating the accurate representation of the series effects (i.e., resistance and inductance). In this regard, the current distribution effects, including the skin and proximity effects, are taken into consideration. The analysis starts with the premise that an accurate modeling of the metal losses in microwave transmission lines for IC and PCB technologies, including frequency-dependent current distribution effects is essential to predict the actual response of a whole transmission channel. From the resulting analysis, three frequency regions were identified, mathematical expressions that relate the cross-section and the electrical characteristics of the structure were developed, and the corresponding accuracy is verified through comparisons with simulations carefully performed on full-wave solvers. Furthermore, dominant effects within different frequency ranges were identified, which allows for the development of new methodologies for the electrical characterization of dielectric materials, and pad de-embedding. All the models and methodologies were applied to single-ended and differential microstrips and striplines, as well as to metal-insulator-metal (MIM) test fixtures used for the characterization of thin dielectric films. In this regard, special attention was paid to appropriately consider the correct propagating modes occurring in the analyzed structures, and also to the scalability of the models for R and L.