A method of tracking assets comprising: a. associating a tracking device (100, 205, 415) with an asset in need of tracking, wherein the tracking device (100, 205, 415) comprises: i. computer memory configured to store a plurality of data messages, each message embodying message information, wherein the memory has a storage capacity; ii. a global positioning system receiver configured to receive GPS information from a GPS satellite network; iii. a cellular signal transmitter (230, 245) configured to transmit data messages to a cellular signal receiver; iv. a motion sensor configured to generate one or more of speed and acceleration data associated with tracking device (100, 205, 415) movement information; and v. a microprocessor in communication with the GPS receiver, the motion sensor and the cellular signal transmitter (230, 245); b. commencing a tracking event upon activation of the tracking device; c. transmitting data messages comprising at least the GPS information and tracking device (100, 205, 415) movement information to the cellular signal receiver when the device is in communication with the receiver; d. storing information generated by the tracking device (100, 205, 415) as data messages during a period when the tracking device (100, 205, 415) is not in communication with the cellular signal receiver, wherein storage of each data message reduces an available portion of the storage capacity in the computer memory; e. comparing a plurality of the stored data messages to at least one of a plurality of retention criteria for determining disposability or non-disposability of the plurality of stored data messages; f. designating at least a first one of the plurality of the stored data messages as "non-disposable" in response to the comparison indicating that the message information of the first stored data message meets at least one of the plurality of retention criteria, thereby identifying the stored data message as a message of interest; g. designating at least a second one of the plurality of the stored data messages as "disposable" in response to the comparison indicating that the message information of the second stored data message does not meet any of the plurality of retention criteria; and h. if the available portion of the storage capacity of the computer memory has reached a predefined threshold, deleting one or more of the stored disposable data messages. a. associating a tracking device (100, 205, 415) with an asset in need of tracking, wherein the tracking device (100, 205, 415) comprises: i. computer memory configured to store a plurality of data messages, each message embodying message information, wherein the memory has a storage capacity; ii. a global positioning system receiver configured to receive GPS information from a GPS satellite network; iii. a cellular signal transmitter (230, 245) configured to transmit data messages to a cellular signal receiver; iv. a motion sensor configured to generate one or more of speed and acceleration data associated with tracking device (100, 205, 415) movement information; and v. a microprocessor in communication with the GPS receiver, the motion sensor and the cellular signal transmitter (230, 245); i. computer memory configured to store a plurality of data messages, each message embodying message information, wherein the memory has a storage capacity; ii. a global positioning system receiver configured to receive GPS information from a GPS satellite network; iii. a cellular signal transmitter (230, 245) configured to transmit data messages to a cellular signal receiver; iv. a motion sensor configured to generate one or more of speed and acceleration data associated with tracking device (100, 205, 415) movement information; and v. a microprocessor in communication with the GPS receiver, the motion sensor and the cellular signal transmitter (230, 245); b. commencing a tracking event upon activation of the tracking device; c. transmitting data messages comprising at least the GPS information and tracking device (100, 205, 415) movement information to the cellular signal receiver when the device is in communication with the receiver; d. storing information generated by the tracking device (100, 205, 415) as data messages during a period when the tracking device (100, 205, 415) is not in communication with the cellular signal receiver, wherein storage of each data message reduces an available portion of the storage capacity in the computer memory; e. comparing a plurality of the stored data messages to at least one of a plurality of retention criteria for determining disposability or non-disposability of the plurality of stored data messages; f. designating at least a first one of the plurality of the stored data messages as "non-disposable" in response to the comparison indicating that the message information of the first stored data message meets at least one of the plurality of retention criteria, thereby identifying the stored data message as a message of interest; g. designating at least a second one of the plurality of the stored data messages as "disposable" in response to the comparison indicating that the message information of the second stored data message does not meet any of the plurality of retention criteria; and h. if the available portion of the storage capacity of the computer memory has reached a predefined threshold, deleting one or more of the stored disposable data messages.