The current chapter covered the fundamental data structures built-in in R, and also covered the concepts of lists and their implementation in R. The chapter introduced built-in data types in R such as vector, and element data types. Also, object-based programming, including S3, S4 and reference classes was introduced. The chapter also introduced one of the most fundamental data structures, link list, and its different variation, such as circular link list and array-based list. The next chapter will introduce stacks and queues data structure.