A non-invasive wireless communication system for monitoring parameters existing within the casing annuli of a subsea hydrocarbon production system. The subsea hydrocarbon production system includes a wellhead housing mounted at the upper end of a well bore and a number of concentric well casings extending from the wellhead housing through the well bore, and the casing annuli are formed between successive ones of the wellhead housing and the well casings The monitoring system comprises an interrogation package which is adapted to wirelessly transmit and receive NFM and/or inductive signals and at least one sensing package which is located in one of the casing annuli and is adapted to wirelessly receive the signals from and transmit response signals to the interrogation package.