Abstract: A method of assessing transaction-level interoperability over a Tactical Data Link (TDL) (104) is provided. The method includes obtaining (202) transaction model data for a platform (102) and/or a TDL standard (105), the data representing a plurality of transactions and a plurality of messages that stimulate particular said transactions. Message data representing a message to be transferred over the TDL is obtained and then the message data is used (206A) with the transaction model data to assess which of the plurality of transactions the message represented by the message data will stimulate according to the transaction model. The method may also/alternatively obtain (204B) transaction data representing a desired transaction to be stimulated from amongst the plurality of transactions, and then use (206B) the transaction data with the transaction model data to assess which of the plurality of messages will stimulate the desired transaction according to the transaction model.