Understanding pressure and pressure relationships is the key to safe well control. Yet, to date, the primary focus of well control has centred on recovery rather than prevention. Incidents related to loss of well control largely occur when the primary barrier, hydrostatic pressure from the drilling fluid, fails to prevent an influx; thus requiring the secondary barrier, closing of the BOPs, to engage in order stop the breach from becoming a full-blown, well control incident. Influxes occur, most often, during the tripping operation where the swab effect lowers the bottomhole pressure below the formation's pressure and can be commonly misidentified when "wellbore breathing," nuisances gases, or cement setting are involved. Influxes can also occur when drilling into unexpected, higher-pressure zones.