The book "Delay Differential Equations and Applications" offers a comprehensive collection of materials used during the school on delay differential equations and their applications held in 2002 in Marrakech. This school, which took place at Cadi Ayyad University, brought together 47 participants and 15 lecturers from 21 countries to engage with the fundamentals and advanced concepts of delay differential equations. The contents of the book are designed to support mathematicians who wish to expand or deepen their knowledge of delay differential equations. It covers both basic and complex topics, including classical approaches to solving these equations. The structure of the book reflects the teaching methods of the school, which consisted of lectures, courses, and communication sessions, thus providing a valuable resource for students and professionals in the fields of mathematics and natural sciences.