A conc-tree is a data-structure that stores element sequences, and provides amortized O(1) time append and prepend operations, O(log n) time insert and remove operations and O(log n) time concatenation. This data structure is particularly viable for functional task-parallel and data-parallel programming, and is relatively simple to implement compared to other data-structures with similar asymptotic complexity. Conc-trees were designed to improve efficiency of data-parallel operations that do not require sequential left-to-right iteration order, and improve constant factors in these operations by avoiding unnecessary copies of the data. Orthogonally, they are used to efficiently aggregate data in functional-style task-parallel algorithms, as an implementation of the conc-list data abstract