Block-STM is a novel parallel execution engine designed to enhance smart contract processing by leveraging a pre-ordered block execution model combined with optimistic Software Transactional Memory (STM). Emerging from the Diem project, this technology allows for accelerated transaction validation on existing blockchain infrastructures without requiring modifications to the validator nodes. Building on ideas from distributed database projects like Calvin and Bohm, Block-STM simplifies concurrency management by executing transactions in blocks, ensuring deterministic outcomes that mirror sequential executions. It employs a "speculate-validate-redo" approach, where transactions are executed optimistically in parallel and then validated, with re-execution occurring if validations fail. Key innovations include task stealing and an ABORTED tag for simple dependency tracking, which reduce unnecessary re-executions and enable near-optimal scheduling. Block-STM's simplicity and efficiency have shown significant performance improvements, achieving over 17x speedup on 32 cores under low/modest contention, and have been integrated into the Diem blockchain core.