  Encoding information in the time-frequency domain is demonstrating its
potential for quantum information processing. It offers a novel scheme for
communications with large alphabets, computing with large quantum systems, and
new approaches to metrology. It is then crucial to secure full control on the
generation of time-frequency quantum states and their properties. Here, we
present an overview of the theoretical background and the technical aspects
related to the characterization of time-frequency properties of two-photon
states. We provide a detailed account of the methodologies which have been
implemented for measuring frequency correlations and for the retrieval of the
full spectral wavefunction. This effort has benefited enormously from the
adaptation of classical metrology schemes to the needs of operating at the
single-photon level.
