Heavy metal contamination poses a significant threat to environmental and human health due to its persistence, bioaccumulation, and toxicity. Conventional monitoring techniques, such as atomic absorption spectroscopy and inductively coupled plasma mass spectrometry, offer high sensitivity but are often expensive, time-consuming, and require sophisticated instrumentation. In recent years, biosensors and nanosensors have emerged as efficient alternatives for real-time, cost-effective, and highly sensitive detection of heavy metals in water, soil, and biological systems. Biosensors, employing biomolecules such as enzymes, DNA, and microbial cells, provide specificity and eco-friendliness, while nanosensors, utilizing nanomaterials like metal nanoparticles, quantum dots, and carbon-based nanostructures, enhance signal transduction and detection limits. This review explores the advancements in bio- and nanosensor technologies, their working principles, sensitivity, selectivity, and applicability in detecting heavy metals like Pb, Cr As, Cd, and Hg. Furthermore, it highlights recent innovations, challenges, and future prospects in developing portable and field-deployable sensor-based monitoring systems for environmental safety and regulatory compliance.