Many efficient algorithms are based on sorting the input data, because sorting often makes solving the problem easier. This chapter discusses the theory and practice of sorting as an algorithm design tool. Section 4.1 first discusses three important sorting algorithms: bubble sort, merge sort, and counting sort. After this, we will learn how to use the sorting algorithm available in the C++ standard library. Section 4.2 shows how sorting can be used as a subroutine to create efficient algorithms. For example, to quickly determine if all array elements are unique, we can first sort the array and then simply check all pairs of consecutive elements. Section 4.3 presents the binary search algorithm, which is another important building block of efficient algorithms.