This paper presents a method for time-resolved atomic inner-shell spectroscopy using a laser-based sampling system consisting of a few-femtosecond visible light pulse and a synchronized sub-femtosecond soft X-ray pulse. This allows direct observation of the dynamics of core-excited atoms with attosecond resolution. The study measured a lifetime of 7.9 ± 1.0 fs for M-shell vacancies in krypton. Core-excited atoms relax by rearranging their electronic structure, and observing these processes in real time requires a pump pulse to initiate dynamics and a delayed probe pulse to detect transition states. Time-resolved spectroscopy based on this pump-probe approach is now routinely used for tracking atomic motion in molecules with femtosecond laser pulses. Electronic motion in weakly bound (Rydberg) states can be captured with optical or infrared pulses, but the relaxation dynamics of core-excited atoms are out of the reach of femtosecond optical techniques. Excitation of a strongly bound electron from an atomic inner shell leads to an ultrafast rearrangement of the electronic system, resulting in the disappearance of the inner-shell vacancy (core hole). The characteristic time constants of the concomitant electronic dynamics range from a few attoseconds to a few femtoseconds. The lifetime of core holes can be inferred from the energy spectral width of the photons or electrons emitted upon the decay of the excited atomic state. However, energy-domain measurements alone are generally unable to provide detailed and accurate insight into the evolution of multi-electron dynamics. Time-domain access is desirable for this purpose. The recent demonstration of isolated sub-femtosecond soft-X-ray pulses and trains of pulses, along with attosecond sampling of photo-electron wave packets with a few-cycle optical wave, has opened the door to attosecond atomic spectroscopy. Here, these tools and techniques are used to track the rearrangement of the electronic system of a core-excited atom. The core hole is created within less than a femtosecond and its subsequent decay can be observed on a few-femtosecond timescale. The applied technique offers attosecond resolution and will allow time-resolved spectroscopy of a wide range of atomic processes. The decay of atomic core holes is a key process in atomic physics, and the study of this process provides insight into the dynamics of core-excited atoms. The paper presents a detailed analysis of the decay of an M-shell vacancy in krypton, demonstrating the feasibility of time-resolved atomic inner-shell spectroscopy. The measured lifetime of the M-shell vacancy is 7.9 ± 1.0 fs, which is in good agreement with previous energy-domain measurements. The study also shows that time-domain spectroscopy is essential for understanding the complex dynamics of multi-electron systems, particularly in the presence of strong fields. The results highlight the importance of time-resolved spectroscopy in studying atomic dynamics and its potential impact on other fields.This paper presents a method for time-resolved atomic inner-shell spectroscopy using a laser-based sampling system consisting of a few-femtosecond visible light pulse and a synchronized sub-femtosecond soft X-ray pulse. This allows direct observation of the dynamics of core-excited atoms with attosecond resolution. The study measured a lifetime of 7.9 ± 1.0 fs for M-shell vacancies in krypton. Core-excited atoms relax by rearranging their electronic structure, and observing these processes in real time requires a pump pulse to initiate dynamics and a delayed probe pulse to detect transition states. Time-resolved spectroscopy based on this pump-probe approach is now routinely used for tracking atomic motion in molecules with femtosecond laser pulses. Electronic motion in weakly bound (Rydberg) states can be captured with optical or infrared pulses, but the relaxation dynamics of core-excited atoms are out of the reach of femtosecond optical techniques. Excitation of a strongly bound electron from an atomic inner shell leads to an ultrafast rearrangement of the electronic system, resulting in the disappearance of the inner-shell vacancy (core hole). The characteristic time constants of the concomitant electronic dynamics range from a few attoseconds to a few femtoseconds. The lifetime of core holes can be inferred from the energy spectral width of the photons or electrons emitted upon the decay of the excited atomic state. However, energy-domain measurements alone are generally unable to provide detailed and accurate insight into the evolution of multi-electron dynamics. Time-domain access is desirable for this purpose. The recent demonstration of isolated sub-femtosecond soft-X-ray pulses and trains of pulses, along with attosecond sampling of photo-electron wave packets with a few-cycle optical wave, has opened the door to attosecond atomic spectroscopy. Here, these tools and techniques are used to track the rearrangement of the electronic system of a core-excited atom. The core hole is created within less than a femtosecond and its subsequent decay can be observed on a few-femtosecond timescale. The applied technique offers attosecond resolution and will allow time-resolved spectroscopy of a wide range of atomic processes. The decay of atomic core holes is a key process in atomic physics, and the study of this process provides insight into the dynamics of core-excited atoms. The paper presents a detailed analysis of the decay of an M-shell vacancy in krypton, demonstrating the feasibility of time-resolved atomic inner-shell spectroscopy. The measured lifetime of the M-shell vacancy is 7.9 ± 1.0 fs, which is in good agreement with previous energy-domain measurements. The study also shows that time-domain spectroscopy is essential for understanding the complex dynamics of multi-electron systems, particularly in the presence of strong fields. The results highlight the importance of time-resolved spectroscopy in studying atomic dynamics and its potential impact on other fields.