The concept of a fuzzy set was introduced by Zadeh. Since the theory has developed in many directions. The kind of fuzzy set have gained wide recognition as a useful tool in the modeling of some uncertain phenomena. these have numerous applications such as computer science, mathematical applied and medicine. The elements of the fuzzy sets are featured by an additional degree which is called the degree of uncertainty. The aim of this book is to introduce new notions of (fuzzy) n-fold ideals, (fuzzy) n-fold weak ideals in BCK/BCI-algebras and investigate several properties of the foldness...