Metal ions can be beneficial or toxic depending on their identity, oxidation state, and concentration. Therefore, the ability to detect and quantify different types of metal ions using portable sensors or in situ imaging agents is important for better environmental monitoring, in vitro medical diagnostics, and imaging of biological systems. While numerous metal ions in different oxidation states are present in the environment and biological systems, only a limited number of them can be detected effectively using current methods. In this Account, we summarize research results from our group that overcome this limitation by the development of a novel class of activity-based sensors based on metal-dependent DNAzymes, which are DNA molecules with enzymatic activity. First, we have developed an in vitro selection method to obtain DNAzymes from a large DNA library of up to 1015 sequences that can carry out cleavage of an oligonucleotide substrate only in the presence of a specific metal ion with high selectivity. Negative selection steps can further be used to improve the selectivity against potentially competing targets by removing sequences that recognize the competing metal ions. Second, we have developed a patented catalytic beacon method to transform the metal-dependent DNAzyme cleavage reaction into a turn-on fluorescent signal by attaching a fluorophore and quenchers to the DNAzyme complex. Because of the difference in the melting temperatures of DNA hybridization before and after metal-ion-dependent cleavage of the DNAzyme substrate, the fluorophore on the DNA cleavage product can be released from its quenchers to create a turn-on fluorescent signal. Because DNAzymes are easy to conjugate with other signaling moieties, such as gold nanoparticles, lanthanide-doped upconversion nanoparticles, electrochemical agents, and gadolinium complexes, these DNAzymes can also readily be converted into colorimetric sensors, upconversion luminescence sensors, electrochemical sensors, or magnetic resonance contrast agents. In addition to describing recent progress in developing and applying these metal ion sensors for environmental monitoring, point-of-care diagnostics, cellular imaging, and in vivo imaging in zebrafish, we summarize major advantages of this class of activity-based sensors. In addition to advantages common to most activity-based sensors, such as enzymatic turnovers that allow for signal amplification and the use of initial rates instead of absolute signals for quantification to avoid interferences from sample matrices, the DNAzyme-based sensors allow for in vitro selection to expand the method to almost any metal ion under a variety of conditions, negative selection to improve the selectivity against competing targets, and reselection of DNAzymes and combination of active and inactive variants to fine-tune the dynamic range of detection. The use of melting temperature differences to separate target binding from signaling moieties in the catalytic beacon method allows the use of different fluorophores and nanomaterials to extend the versatility and modularity of this sensing platform. Furthermore, sensing and imaging artifacts can be minimized by using an inactive mutant DNAzyme as a negative control, while spatiotemporal control of sensing/imaging can be achieved using optical, photothermal, and endogenous orthogonal caging methods. Finally, current challenges, opportunities, and future perspectives for DNAzymes as activity-based sensors are also discussed.