Cello is a library of succinct data structures, oriented in particular for string searching and other string operations. Usually, searching for patterns in a string takes O(n) time, where n is the length of the string. Indices can speedup the search, but take additional space, which can be costly for very large strings. A data structure is called succinct when it takes n + o(n) space, where n is the space needed to store the data anyway. Hence succinct data structures can provide additional operations with limited space overhead.