Explore the concept of hash trees, also known as Merkle trees, in this comprehensive conference talk from ACCU 2019. Discover how these binary tree structures, proposed by Ralph C. Merkle in 1979, use one-way hash functions to compute parent nodes from child nodes. Learn about the applications of hash trees in building efficient authenticated data structures, where query responses come with proofs of correctness. Gain a quick introduction to hash functions before delving into hash trees and their derived authenticated data structures. This expanded version of a previous "quickie" talk is presented by Ahto Truu, an industry veteran with extensive experience in software development, architecture, and systems analysis, who currently focuses on data integrity preservation at Guardtime.