X-ray emission spectroscopy (XES) is a non-destructive photon-in/photon-out technique that provides exceptional chemical sensitivity to the occupied electronic states of materials. It enables quantitative insights into oxidation states, coordination environments, charge-transfer interactions, and spin states, and has emerged as an indispensable probe for electronic structure characterization across diverse fields, including quantum materials, energy catalysis, and life sciences. With the continuous development of laboratory X-ray sources, synchrotron radiation facilities, and X-ray free-electron lasers, XES methodologies have been progressively refined, evolving toward higher energy resolution and enhanced sensitivity. This review first introduces the fundamental principles of both non-resonant and resonant XES, as well as the categories of electronic structure information that can be extracted from these techniques. It then outlines the development and implementation of laboratory-based and synchrotron-based XES spectrometers, with particular emphasis on resonant inelastic X-ray scattering (RIXS) instrumentation and its applications. At present, both hard and soft X-ray emission spectroscopy have become technologically mature platforms worldwide. In contrast, XES operating in the tender X-ray energy regime(2000—5000 eV) has progressed more slowly due to the special photon energy range, limitations in crystal optics, and the relatively limited availability of synchrotron beamlines in this regime. Meanwhile, the tender X-ray energy range encompasses light elements such as P, S, and Cl, as well as 4d transition metals, which play pivotal roles in catalysis, energy conversion, and biological systems. Accurate characterization of their electronic structures is therefore essential for advancing research in these areas. Motivated by these scientific demands, this review systematically discusses the design principles and geometrical configurations of tender XES spectrometers, analyzes their respective advantages and limitations, and compares different configurations in terms of energy resolution, diffraction efficiency, energy coverage, and experimental compatibility. Furthermore, we comprehensively summarize the technical progress achieved over the past two decades in tender XES instrumentation based on both laboratory and synchrotron radiation sources. This review aims to provide researchers with a deeper understanding of the tender XES methodology and its potential extensions to broader applications, while also laying a foundation for the development and optimization of future XES instrumentation in this energy regime.
In addition, catalytic reactions, biological processes, and electrochemical battery systems typically occur under dynamic conditions, often involving liquid phases, multiphase interfaces, and complex reaction pathways. Considering these characteristics, this review places particular emphasis on the compatibility of tender XES spectrometers with sample environments under in situ/operando conditions. We discuss the development and application of tender XES in situ/operando infrastructure, including helium gloveboxes, dedicated sample chambers, and gas reaction cells. Finally, we examine the remaining technical challenges associated with practical applications of tender XES and outline future developmental directions, together with our perspective on methodological strategies for enabling operando XES in this energy regime.