The Th-229 nucleus is known to possess an isomeric state at an energy of about 8 eV above the ground state,
several orders of magnitude lower than typical nuclear excitation energies. This has inspired a field of
low-energy nuclear physics where nuclear transition rates will be influenced by the electron shell.
The low energy makes the Th-229 isomer accessible to resonant laser excitation. Realized with laser-cooled
trapped thorium ions or with thorium dopant ions in a transparent solid, the nuclear resonance may serve as
the reference for an optical clock of very high accuracy. Precision frequency comparisons between such a
nuclear clock and conventional atomic clocks will provide sensitivity to effects of hypothetical new physics
beyond the standard model. While laser excitation of Th-229 remains an unsolved challenge, recent experiments
have provided essential information on the transition energy and relevant nuclear properties.