AVL (2 scientist guys’ last names: Adelson-Velsky and Landis) Tree is a special BST (binary search tree). It solves the problem of a BST becoming unbalanced, which at its extreme, results in a linked list. It achieves this by recursively asking “Am I balanced?” after a new node is added. If the answer is “NO!” then single or double rotations need to occur depending on the type of imbalance.