A cryotherapy system comprising: a plurality of cryoprobes, each such cyroprobe having a shaft with a closed distal end (104, 604) adapted for insertion into a body and conduits (528, 532) for flowing a cryogenic fluid through the shaft to reduce a temperature of the distal end; a source of the cryogenic fluid; a plurality of flow-control metering valves in fluid communication with the conduits of the plurality of cryoprobes and with the source of the cryogenic fluid; a compressor (154, 628) in fluid communication with the conduits of the plurality of cryoprobes to define a self-contained fluid system; a source of warmed gas in fluid communication with the flow-control metering valves; and a computer processor comprising: instructions to control the plurality of flow-control metering valves and the compressor to provide desired flows of the cryogenic fluid through the conduits of the self-contained fluid system; and instructions to control the flow-control metering valves to initiate flow of the warmed gas through the conduits as part of an active thaw procedure; characterised in that the computer processor further comprises: instructions to control the compressor and the plurality of flow-control metering valves to provide an initial flow of the cryogenic fluid through the conduits of the cryoprobes under physical conditions near a critical point of a liquid-vapor system for the cryogenic fluid, wherein the critical point defines a point in a phase diagram of the liquid-vapor system where molar volumes are substantially equivalent for liquid and gas, whereby vapor lock associated with freezing of the cryoprobes is avoided. a plurality of cryoprobes, each such cyroprobe having a shaft with a closed distal end (104, 604) adapted for insertion into a body and conduits (528, 532) for flowing a cryogenic fluid through the shaft to reduce a temperature of the distal end; a source of the cryogenic fluid; a plurality of flow-control metering valves in fluid communication with the conduits of the plurality of cryoprobes and with the source of the cryogenic fluid; a compressor (154, 628) in fluid communication with the conduits of the plurality of cryoprobes to define a self-contained fluid system; a source of warmed gas in fluid communication with the flow-control metering valves; and a computer processor comprising: instructions to control the plurality of flow-control metering valves and the compressor to provide desired flows of the cryogenic fluid through the conduits of the self-contained fluid system; and instructions to control the flow-control metering valves to initiate flow of the warmed gas through the conduits as part of an active thaw procedure; characterised in that the computer processor further comprises: instructions to control the compressor and the plurality of flow-control metering valves to provide an initial flow of the cryogenic fluid through the conduits of the cryoprobes under physical conditions near a critical point of a liquid-vapor system for the cryogenic fluid, wherein the critical point defines a point in a phase diagram of the liquid-vapor system where molar volumes are substantially equivalent for liquid and gas, whereby vapor lock associated with freezing of the cryoprobes is avoided. instructions to control the plurality of flow-control metering valves and the compressor to provide desired flows of the cryogenic fluid through the conduits of the self-contained fluid system; and instructions to control the flow-control metering valves to initiate flow of the warmed gas through the conduits as part of an active thaw procedure; characterised in that the computer processor further comprises: instructions to control the compressor and the plurality of flow-control metering valves to provide an initial flow of the cryogenic fluid through the conduits of the cryoprobes under physical conditions near a critical point of a liquid-vapor system for the cryogenic fluid, wherein the critical point defines a point in a phase diagram of the liquid-vapor system where molar volumes are substantially equivalent for liquid and gas, whereby vapor lock associated with freezing of the cryoprobes is avoided. instructions to control the compressor and the plurality of flow-control metering valves to provide an initial flow of the cryogenic fluid through the conduits of the cryoprobes under physical conditions near a critical point of a liquid-vapor system for the cryogenic fluid, wherein the critical point defines a point in a phase diagram of the liquid-vapor system where molar volumes are substantially equivalent for liquid and gas, whereby vapor lock associated with freezing of the cryoprobes is avoided.