Attosecond science is well developed for atoms and promising results have been obtained for molecules and solids. Here, we review the first steps in developing attosecond time-resolved measurements in liquids. These advances provide access to time-domain studies of electronic dynamics in the natural environment of chemical reactions and biological processes. We concentrate on two techniques that are representative of the two main branches of attosecond science: pump-probe measurements using attosecond pulses and high-harmonic spectroscopy (HHS). In the first part, we discuss attosecond photoelectron spectroscopy with cylindrical microjets and its application to measure time delays between liquid and gaseous water. We present the experimental techniques, the new data-analysis methods and the experimental results. We describe in detail the conceptual and theoretical framework required to fully describe attosecond chronoscopy in liquids at a quantum-mechanical level. This includes photoionization delays, scattering delays, as well as a coherent description of electron transport and (laser-assisted) photoemission and scattering. As a consequence, we show that attosecond chronoscopy of liquids is, in general, sensitive to both types of delays, as well as the electron mean-free paths. Through detailed modeling, involving state-of-the-art quantum scattering and Monte-Carlo trajectory methods, we show that the photoionization delays dominate in attosecond chronoscopy of liquid water at photon energies of 20-30 eV. This conclusion is supported by a near-quantitative agreement between experiment and theory. In the second part, we introduce HHS of liquids based on flat microjets. These results represent the first observation of high-harmonic generation (HHG) in liquids extending well beyond the visible into the extreme-ultraviolet regime.
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