This dissertation aims to evaluate the package effects on the performance of radio frequency integrated circuits (RFICs) for wireless applications. A model-based study is presented to compare the effects between flip-chip and wire-bond packages on a front-end cascode low-noise amplifier (LNA) in a 2.45 GHz CMOS wireless local area network (WLAN) receiver. To construct the package electrical models, specific modeling dies are designed to help extract the equivalent-circuit elements from measured S-parameters for chip-package interconnects. Furthermore, the ground-proximity effect on on-chip spiral inductors in a flip-chip package is first observed and presented in this modeling study. Excellent agreement between modeling and measurement is obtained by up to 20 GHz for a 64-pin flip-chip ball grid array (FC-BGA) package and a 64-pin wire-bond quad flat nonlead (WB-QFN) package. For practical applications, the established package models are used to predict the degradation of the figure of merit for the cascode LNA under packaged condition. Chip-package co-simulations can achieve good agreement with measurements, and thus can persuasively account for the complete effects caused by the two different packages on the cascode LNA. To simultaneously consider the package and board interconnect effects on RFICs, this dissertation also designs and implements a 1.95 GHz upconverter for the wideband code-division multiple-access (W-CDMA) transmitter. Specific ground wire-bonding and board connection are designed to minimize the linearity degradation due to package and board interconnects. Nonlinear analysis technique is also used to evaluate the nonlinear distortion of the upconverter in the chip-package-board co-design phase. The final measurement results have successfully verified the co-design predictions and simulations for this upconverter.