VATA: A library for efficient manipulation of non-deterministic tree automata. In this paper, we present VATA, a versatile and efficient open-source tree automata library applicable, e.g., in formal verification. The library supports both explicit and semi-symbolic encoding of non-deterministic finite tree automata and provides efficient implementation of standard operations on both. The semi-symbolic encoding is intended for tree automata with large alphabets. For storing their transition functions, a newly implemented MTBDD library is used. In order to enable the widest possible range of applications of the library even for the semi-symbolic encoding, we provide both bottom-up and top-down semi-symbolic representations. The library implements several highly optimised reduction algorithms based on downward and upward simulations as well as algorithms for testing automata inclusion based on upward and downward antichains and simulations. We compare the performance of the algorithms on a set of test cases and we also compare the performance of VATA with our previous implementations of tree automata.