The volume "9th Congress on Electronic Structure: Principles and Applications (ESPA 2014)" brings together the research findings presented at the 9th International Conference on Electronic Structure, which took place from July 2 to 4, 2014, in Badajoz, Spain. The contributions cover a wide range of topics, including theoretical foundations, methods, materials science, chemical structure and reactivity, as well as environmental effects and modeling. These outstanding works, originally published in the journal "Theoretical Chemistry Accounts," are now available in a hardcover format as well as in a special electronic edition. The volume provides valuable content for all researchers in the field of theoretical chemistry and is particularly useful for research teams and libraries with limited access to the journal.