In this thesis we investigate a range of performance evaluation problems encountered in wireless networks. The first part of the thesis covers the performance analysis of ad hoc networks based on Gaussian approximations. More specifically, chapter 3 quantifies the benefits of perfect rate adaptation on the achievable throughput of random ad hoc networks. The advantages of Large Area Synchronous (LAS) spreading codes in ad hoc networks obeying an infinite rectilinear node topology are outline in Chapter 4. The second part of the research covers the performance analysis of wireless systems based on characteristic functions. Specifically, the exact average Bit Error Ratio (BER) calculation of an asynchronous DS-CDMA system using random spreading sequences and BPSK modulation for communicating over various faxing channels is discussed in Chapter 5. The exact average BER calculation of general Rectangular Adaptive Quadrature Amplitude Modulation (R-QAM) systems subjected to asynchronous Co-Channel Interference (CCI) and Nakagami-m fading is the topic of Chapter 6. The exact average BER calculation of a BPSK- or QPSK-modulated Orthogonal Frequency Division Multiplexing (OFDM) system subjected to Carrier Frequency Offset (CFO), Phase Estimation Error (PER) and Nakagami-m fading is the topic of Chapter 7.