A method of maintaining a thermal history of samples stored in a temperature-controlled sample storage system, the system including one or more containers each storing one of the samples; one or more storage objects each housing one or more containers or other storage objects ; one or more structures defining a temperature-controlled storage environment and housing the one or more storage objects ; machine readable tags each of which is associated with a separate container and storage object , 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 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 housing a particular container within a specific structure; c) locating by the operator one of the specific storage objects housing the particular container by reading the identification code of the machine-readable tag associated with that specific storage object; d) reading by the operator the value of the temperature-dependant characteristic of the machine-readable tag associated with the specific storage object located in step c); e) storing the read identification code and value of the temperature-dependant characteristic in the database ; f) removing by the operator of the particular container from a storage object located in step b), or another of the specific storage objects from the temperature controlled storage environment; and g) returning by the operator the one or more specific storage objects to the specific structure ; wherein each machine readable tag includes a plurality of resonant members encoding an identification code and at least one of the resonant members has the temperature-dependant characteristic. a) monitoring the temperature in each temperature-controlled storage environment 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 housing a particular container within a specific structure; c) locating by the operator one of the specific storage objects housing the particular container by reading the identification code of the machine-readable tag associated with that specific storage object; d) reading by the operator the value of the temperature-dependant characteristic of the machine-readable tag associated with the specific storage object located in step c); e) storing the read identification code and value of the temperature-dependant characteristic in the database ; f) removing by the operator of the particular container from a storage object located in step b), or another of the specific storage objects from the temperature controlled storage environment; and g) returning by the operator the one or more specific storage objects to the specific structure ; wherein each machine readable tag includes a plurality of resonant members encoding an identification code and at least one of the resonant members has the temperature-dependant characteristic. <EOS>