This book presents a modern approach to evaluating subgrade and base layer performance in road construction using the Repeated Lightweight Deflectometer (RLWD). It explains how rapid in-situ measurements of stiffness and deformation can improve pavement design, providing a practical alternative to traditional laboratory testing methods. The book provides a comprehensive methodology for applying RLWD testing in mechanistic-empirical pavement design. It introduces the theoretical background of resilient modulus and permanent deformation behavior of geomaterials, followed by detailed guidance on RLWD testing procedures, data interpretation, and validation. Comparisons with traditional Repeated Load Triaxial (RLT) tests demonstrate the reliability of RLWD-derived stiffness and deformation parameters. The book also explains how RLWD measurements can be integrated into mechanistic-empirical pavement design frameworks to evaluate stresses, strains, and long-term pavement performance. Case studies illustrate the application of RLWD for different soils and unbound materials under varying stress and moisture conditions. In addition, the influence of environmental factors, field variability, and construction practices on pavement performance is discussed, showing how RLWD testing can support real-time quality control, more realistic design inputs, and sustainable road infrastructure management. The book is intended for pavement engineers, geotechnical engineers, transportation researchers, and infrastructure consultants involved in road design and construction. It will also serve graduate students and university instructors in civil engineering, as well as public-sector agencies and laboratories responsible for pavement evaluation, materials testing, and infrastructure management and maintenance.