is ≤50 + sum of txn fees • Only need to validate txns that are not already in mempool • If you don’t have listed transaction, you’ll need to request it from the node that sent block. • If you’re unable to find txn or if it’s already in UTXO set, it’s invalid. Discard block. • Txns can only spend from UTXOs or from other txns in same block; otherwise, block is invalid. 17. Validating Block Txns