In this book, formulation and validation of some computer-programs such as GRIC1-toolkit, GRIC2-toolkit and BUF-toolkit have been described. These programs are useful in the investigation of gamma-ray shielding behaviour of engineering materials. Detailed literature concerned with the measurements of each Gamma-rays shielding parameter (GSP) has been systematically provided in the book. Novel concepts of optimum-thickness value and Air-Slab correction have been discussed in detail. Gamma-rays shielding behaviours (GSBs) of some building materials as Single Layer Homogeneous Shield (SLHS) and Double Layered- Heterogeneous-Shield (DLHS) has been provided with some useful conclusions. During the compilation of ANS-standards (ANS-6.4.3, 1991), Coherent-scattering and Bremsstrahlung phenomena have been neglected. Recently, more accurate values of GSPs have been obtained thus the ANS-standards should be updated in near future. This book will be useful in this revision particularly for Double Layered Transmission Exposure Buildup Factors.