Time-varying photonics offers a route to controlling light through temporal, rather than solely spatial, variations of the optical environment. Low-index transparent conducting oxides have emerged as a particularly powerful platform for accessing this regime. When excited near their crossover wavelength, these materials’ refractive index can undergo exceptionally large changes on femtosecond timescales [1]. The resulting temporal gradients allow propagating photons to exchange both energy and momentum with the evolving medium, introducing new degrees of freedom for manipulating optical fields beyond the capabilities of static photonic structures [2]. Understanding these strongly driven systems also requires moving beyond conventional material descriptions, as their ultrafast response originates from the coupled dynamics of the electron plasma and the solid-state lattice under broadband excitation outside the perturbative approach [3]. This presentation will review the development of this research from the emergence of low-index ultrafast photonics to current realizations of time-varying optical systems, and will discuss future opportunities enabled by this approach. Recent advances include ultrafast pulse characterization, quantum-dot emission enhancement, all-optical polarization control, spatio-temporal photonics, and machine-learning-assisted ultrafast photonics [4–7], pointing towards applications in nonlinear and integrated photonics, ultrafast information processing, and quantum science.