Fiber-matrix adhesion is a variable to be optimized in order to get the best properties and performance in composite materials. This article schematically illustrates fiber matrix interphase for composite materials. It discusses thermodynamics of interphase in terms of surface energy, contact angle, work of adhesion, solid surface energy, and wetting and wicking. The article describes the change in interphase depending on the reinforcing fiber such as glass fiber, polymeric fiber, and carbon fiber. It discusses fiber-matrix adhesion measurements by direct methods, indirect methods, and composite laminate tests. The effects of interphase on fiber-matrix adhesion are also discussed. The article concludes with a discussion on interphase and fiber-matrix adhesion effects on composite mechanical properties, such as composite on-axis properties, composite off-axis properties, and composite fracture properties.