The success of strontium-based optical lattice clocks in the last five years has led to a recommendation by BIPM of strontium as a future standard of frequency and time. Due to the excellent agreement between three international labs, the strontium optical clock transition is the best agreed-upon optical frequency to date. We use the international optical clock data to limit present-day drift of fundamental constants and their coupling to the ambient gravitational potential. Strontium lattice clocks are still making rapid progress and promise a large signal-to-noise improvement over single-ion-based frequency standards by employing O(10⁴) atoms. Reaching quantum-projection-noise limited measurement requires a careful study and control of the many-body interactions in the system. We measure interactions between ultracold fermions at the 10⁻¹⁷ level and relate them to s-wave collisions due to a loss of indistinguishability during the spectroscopic process. This new understanding of the many-body effects will increase the precision of current optical lattice clock systems and can lead to the accuracy level that has so far been pioneered only in single particle (trapped ion) systems. A second generation strontium system is used to control ultracold interactions in an otherwise ideal gas of bosonic ⁸⁸Sr via the optical Feshbach resonance effect. These new measurement and control capabilities pave the way to reach atomic shot-noise limited optical clock performance without detrimental effects from large atom numbers.