The goal of this textbook is easy to understand explanation of the fundamentals and philosophy of ISO OSI approach to the standardization of computer communication. The text deals with higher layers of the ISO OSI model. The transport, session, presentation, and application layers are discussed step-by-step in the first half of the book. The second half of the book is devoted to the specific applications – FTAM (File Transfer, Access and Management), Message Handling System X.400 and Directory Services X.500. This book may help significantly in understanding the comprehensive, formal texts of official ISO OSI documents.