We report optical measurements demonstrating that the low-energy relaxation rate (1/τ) of the conduction electrons in Sr 2 RuO 4 obeys scaling relations for its frequency (ω) and temperature (T ) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, 1/τ ∝ (ħ hω) 2 + (pπk B T ) 2 with p = 2, and ω/T scaling applies. Many-body electronic structure calculations using dynamical mean-field theory confirm the low-energy Fermi-liquid scaling, and provide quantitative understanding of the deviations from Fermi-liquid behavior at higher energy and temperature. The excess optical spectral weight in this regime provides evidence for strongly dispersing "resilient" quasiparticle excitations above the Fermi energy. PACS numbers: 78.47.db, 71.10.Ay, 72.15.Lh, 74.70.Pq Liquids of interacting fermions yield a number of different emergent states of quantum matter. The strong correlations between their constituent particles pose a formidable theoretical challenge. It is therefore remarkable that a simple description of low-energy excitations of fermionic quantum liquids could be established early on by Landau [1], in terms of a dilute gas of "quasiparticles" with a renormalized effective mass, of which 3 He is the best documented case [2, 3].Breakdown of the quasiparticle concept can be observed in the transport of metals tuned onto a quantum phase transition, but Fermi-liquid (FL) behavior is retrieved away from the quantum-critical region [4, 5]. The relevance of FL theory to electrons in solids is documented by a number of materials, such as transition metals [6], heavy-fermion compounds [7], and doped semiconductors [8]. Among transition-metal oxides, Sr 2 RuO 4 is a remarkable example which has been heralded as the solid-state analogue of 3 He [9] for at least three reasons: remarkably large and clean monocrystalline samples can be prepared, transport properties display low-temperature FL characteristics [10], and there is evidence for p-wave symmetry of its superconducting phase [11], as in superfluid 3 He.FL theory makes a specific prediction for the universal energy and temperature dependence of the inelastic lifetime of quasiparticles: Because of phase-space constraints imposed by the Pauli principle as well as momentum and energy conservation, it diverges as 1/ω 2 or 1/T 2 [1, 5]. More precisely, the inelastic optical relaxation rate is predicted to vanish according to the scaling law 1/τ ∝ (ħ hω) 2 + (pπk B T ) 2 , with p = 2 [12][13][14]. This leads to universal ω/T scaling of the optical conductivity σ(ω) in the thermal regime ħ hω ∼ k B T [14]. Surprisingly however, despite almost 60 years of research on Fermi liquids, this universal behavior of the optical response, and especially the specific statistical factor p = 2 relating the energy and temperature dependence have not yet been established experimentally [13][14][15][16][17].Here, we report optical measurements of Sr 2 RuO 4 with 0.1 meV resolution [18,19] which reveal this universal FL scaling law [20]. We establish experiment...