This chapter presents an exploration of intelligent reflecting surface (IRS) assisted wireless communications, delving into both its design principles and an in-depth analysis of its performance. The chapter begins with an introduction to IRS, detailing its fundamental principles and role in enhancing wireless communication systems. Subsequently, a section on “Design, Implementation, and Real-world Considerations” delves into the architecture of a typical IRS setup, various methods for IRS reconfiguration, practical reflection models employed by IRS, and considerations regarding the power consumption of IRS. Following this, the chapter presents a comprehensive analysis of the performance of a single-IRS-aided single-input single-output system in Rayleigh fading channels. This analysis includes the derivation of approximate outage probability (OP) expressions, asymptotic OP analysis, and diversity order analysis based on asymptotic OP. Additionally, approximate average symbol error rate (ASER) expressions are derived for both coherent and non-coherent modulation schemes, along with asymptotic ASER analysis and diversity order analysis based on asymptotic ASER. In addition, the chapter also includes a presentation of numerical and simulation results, accompanied by a detailed discussion of their implications.