The paper discusses the concept of Coherent Perfect Absorbers (CPAs), which are optical systems that can perfectly absorb incident light at discrete frequencies under specific conditions. The authors demonstrate that by adding a precise amount of dissipation to a medium under coherent monochromatic illumination, the elastic S-matrix can be manipulated to move a zero onto the real wavevector axis, effectively achieving perfect absorption. This process is the time-reversed version of lasing at threshold. The effect is demonstrated in a simple silicon slab geometry illuminated in the 500-900 nm range. CPAs are novel linear optical elements and absorptive interferometers, potentially useful for controlled optical energy transfer. The paper also explores the analytic properties of the S-matrix and the role of interference in the CPA mechanism, showing that perfect absorption occurs when the reflected and transmitted parts of the incident beams interfere destructively. The authors discuss the conditions for achieving high absorption contrasts and the potential applications of CPAs in various optical systems.The paper discusses the concept of Coherent Perfect Absorbers (CPAs), which are optical systems that can perfectly absorb incident light at discrete frequencies under specific conditions. The authors demonstrate that by adding a precise amount of dissipation to a medium under coherent monochromatic illumination, the elastic S-matrix can be manipulated to move a zero onto the real wavevector axis, effectively achieving perfect absorption. This process is the time-reversed version of lasing at threshold. The effect is demonstrated in a simple silicon slab geometry illuminated in the 500-900 nm range. CPAs are novel linear optical elements and absorptive interferometers, potentially useful for controlled optical energy transfer. The paper also explores the analytic properties of the S-matrix and the role of interference in the CPA mechanism, showing that perfect absorption occurs when the reflected and transmitted parts of the incident beams interfere destructively. The authors discuss the conditions for achieving high absorption contrasts and the potential applications of CPAs in various optical systems.