This is a book about high energy physics. It is supervised by the professors of nuclear physics at faculty of Science, helwan university, Cairo, Mohammed El-Sayed El-Nagdy, Abd Allah Abd El-Salam Mohammed, Physics Department, Faculty of Science, Cairo University and Badawy Mohammed Badawy Assistant Proffessor of Nuclear Physics, Reactor Physics Department, Atomic Energy Authority. This book is concerned with the study of Si-Em interactions at 3.7A GeV incident energy, a study was done to examine the effect of the projectile size on the multiplicity characteristics of the pions emitted in the forward (FHS) and backward (BHS) directions. In this study it is found that the projectile mass is the main parameters participating in the production process of those forward emitted pions, on the other hand it is found that the multiplicity of the backward emitted pions exhibits a universality in their behavior, independent of the projectile size and may give a good prediction with the modified cascade model (MCM).