This chapter addresses the fundamental challenges of noise and interference in wireless communication systems. It explores how both inherent thermal noise and external sources limit the ability to communicate, and introduces key parameters such as noise factor, noise figure, receiver sensitivity and noise rise. Further, this chapter discusses the concepts of signal-to-noise (SNR) and signal-to-interference-plus-noise ratio (SINR) and their impact on data rates. Additionally, it provides a step by step introduction to link budget calculation. Finally, it examines how non-linear components within the system can generate intermodulation products and harmonics, adding to the complexity of the communication channel.