A method of maintaining a thermal history of samples stored in a temperature-controlled sample storage system, the system including one or more containers (210, 212, 214, 216, 522, 602, 632) each storing one of the samples; one or more storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) each housing one or more containers or other storage objects (210, 212, 214, 216, 522, 602, 632); one or more structures (200, 202, 204, 206, 512, 618) defining a temperature-controlled storage environment and housing the one or more storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628); machine readable tags (14, 222, 224, 524, 526, 528, 530, 532, 534, 536, 604, 634) each of which is associated with a separate container and storage object (208, 510, 514, 516, 518, 520, 610, 614, 628), each tag encoding an identification code and having a temperature-dependant characteristic; and an interrogator for reading the identification code and the temperature-dependant characteristic, the method including the steps of: a) monitoring the temperature in each temperature-controlled storage environment (200, 202, 204, 206) in steady-state condition using a permanently mounted temperature sensor to provide a continuous temperature reading; b) interrogating a database by an operator to determine one or more specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) housing a particular container (210, 212, 214, 216, 522, 602, 632) within a specific structure; c) locating by the operator (700, 702, 704) one of the specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) housing the particular container (210, 212, 214, 216, 522, 602, 632) by reading the identification code of the machine-readable tag (14, 222, 224) associated with that specific storage object; d) reading by the operator (700, 702, 704) the value of the temperature-dependant characteristic of the machine-readable tag (14, 222, 224) associated with the specific storage object (208, 510, 514, 516, 518, 520, 610, 614, 628) located in step c); e) storing (706, 708) the read identification code and value of the temperature-dependant characteristic in the database; f) removing by the operator (700, 702, 704) of the particular container (210, 212, 214, 216, 522, 602, 632) from a storage object located in step b), or another of the specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) from the temperature controlled storage environment; and g) returning by the operator (704, 706) the one or more specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) to the specific structure (200, 202, 204, 206, 512, 618); wherein each machine readable tag (14, 222, 224) includes a plurality of resonant members encoding an identification code (16, 18, 20, 22, 24, 26, 28, 30, 32, 34) and at least one of the resonant members (16, 18, 20, 22, 24, 26, 28, 30, 32, 34) has the temperature-dependant characteristic. a) monitoring the temperature in each temperature-controlled storage environment (200, 202, 204, 206) in steady-state condition using a permanently mounted temperature sensor to provide a continuous temperature reading; b) interrogating a database by an operator to determine one or more specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) housing a particular container (210, 212, 214, 216, 522, 602, 632) within a specific structure; c) locating by the operator (700, 702, 704) one of the specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) housing the particular container (210, 212, 214, 216, 522, 602, 632) by reading the identification code of the machine-readable tag (14, 222, 224) associated with that specific storage object; d) reading by the operator (700, 702, 704) the value of the temperature-dependant characteristic of the machine-readable tag (14, 222, 224) associated with the specific storage object (208, 510, 514, 516, 518, 520, 610, 614, 628) located in step c); e) storing (706, 708) the read identification code and value of the temperature-dependant characteristic in the database; f) removing by the operator (700, 702, 704) of the particular container (210, 212, 214, 216, 522, 602, 632) from a storage object located in step b), or another of the specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) from the temperature controlled storage environment; and g) returning by the operator (704, 706) the one or more specific storage objects (208, 510, 514, 516, 518, 520, 610, 614, 628) to the specific structure (200, 202, 204, 206, 512, 618); wherein each machine readable tag (14, 222, 224) includes a plurality of resonant members encoding an identification code (16, 18, 20, 22, 24, 26, 28, 30, 32, 34) and at least one of the resonant members (16, 18, 20, 22, 24, 26, 28, 30, 32, 34) has the temperature-dependant characteristic.