The principles of orthogonal frequency division multiplexing (OFDM) modulation have been in existence for several decades. However, in recent years these techniques have quickly moved out of textbooks and research laboratories and into practice in modern communications systems. The techniques are employed in data delivery systems over the phone line, digital radio and television, and wireless networking systems. What is OFDM? And why has it recently become so popular? This article will review the fundamentals behind OFDM techniques, and also discuss common impairments and how, in some cases, OFDM mitigates their effect. Where applicable, the impairment effects and techniques will be compared to those in a single carrier system. A brief overview of some modern applications will conclude the article.