Contaminants, such as nucleic acids or toxic small molecules, threaten both human health and ecosystems when they infiltrate the environment. The precise and highly sensitive identification of such contaminants holds paramount importance across diverse domains, including safeguarding food integrity, facilitating clinical diagnostics, and monitoring environmental conditions. Traditional methodologies, encompassing spectroscopy, chromatography, sequencing, and metagenomics, have conventionally served pivotal roles in detection processes. Nevertheless, these methods have encountered recurring challenges related to sensitivity, specificity, and portability. This review focuses on the groundbreaking CRISPR/Cas12-based biosensors. These biosensors leverage the incredible precision and programmability of the CRISPR/Cas system to recognize specific targets. Here, we comprehensively assess the fundamental mechanisms that enable specific and sensitive detection, ranging from guide RNA design to collateral cleavage. The versatility of CRISPR/Cas12 biosensors becomes evident through their diverse applications. These applications encompass medical diagnostics, food safety, and environmental monitoring. The transition from conventional detection methods to biosensors and ultimately to CRISPR/Cas12 biosensors represents a significant milestone in toxic contaminant detection. By incorporating molecular biology, nanotechnology, and bioinformatics, these biosensors have the potential to reshape the landscape of water safety, diagnostics, and environmental monitoring. CRIPSR-Cas diagnostics is a transformative technology that paves the way for a safer and healthier future for the environment and human life.