This review focuses on the development and application of fluorescent sensors for measuring metal ions in living systems. It begins by highlighting the critical role of metals in biological processes and the need for precise visualization and quantification of metal ion distribution within cells. The introduction discusses the limitations of traditional analytical techniques, such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), which are insufficient for single-cell or subcellular analysis. The review then delves into the principles of fluorescence imaging and sensor design, emphasizing the importance of photophysical properties of fluorophores and the mechanisms by which metal binding alters fluorescence signals. Key mechanisms include energy transfer, electron transfer, and Förster resonance energy transfer (FRET). The review also covers the different classes of sensors, including molecular probes, genetically encoded probes, and hybrid probes, each with unique advantages and applications. Additionally, it addresses important considerations for the introduction of sensors, such as intracellular concentration, buffering, and localization, and provides a historical perspective on the evolution of fluorescent sensors for metal ions. The review concludes with a discussion of recent advances and future prospects in the field, emphasizing the potential of fluorescent sensors to provide insights into cellular metal homeostasis and dynamics.This review focuses on the development and application of fluorescent sensors for measuring metal ions in living systems. It begins by highlighting the critical role of metals in biological processes and the need for precise visualization and quantification of metal ion distribution within cells. The introduction discusses the limitations of traditional analytical techniques, such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), which are insufficient for single-cell or subcellular analysis. The review then delves into the principles of fluorescence imaging and sensor design, emphasizing the importance of photophysical properties of fluorophores and the mechanisms by which metal binding alters fluorescence signals. Key mechanisms include energy transfer, electron transfer, and Förster resonance energy transfer (FRET). The review also covers the different classes of sensors, including molecular probes, genetically encoded probes, and hybrid probes, each with unique advantages and applications. Additionally, it addresses important considerations for the introduction of sensors, such as intracellular concentration, buffering, and localization, and provides a historical perspective on the evolution of fluorescent sensors for metal ions. The review concludes with a discussion of recent advances and future prospects in the field, emphasizing the potential of fluorescent sensors to provide insights into cellular metal homeostasis and dynamics.