Environmental RNA (eRNA) represents a transformative frontier in ecological surveillance, offering unprecedented precision in biodiversity assessment, ecosystem monitoring, and organismal health evaluation. Unlike environmental DNA (eDNA), eRNA’s rapid degradation provides a snapshot of metabolically active organisms, enabling real-time ecological insights. Beyond species detection, eRNA holds the potential to refine taxonomic resolution, identifying life-history stages, sexes, and even phenotype-specific variations within populations. Moreover, transcriptional responses encoded in eRNA offer a novel avenue for noninvasive monitoring of organismal physiology and ecosystem-level health. This chapter explores the untapped potential of eRNA in ecological studies, from the fundamental understanding of its dynamics in environmental systems to cutting-edge molecular methodologies for its detection, quantification, and preservation. We discuss the development of high-specificity eRNA assays that push detection beyond species-level identification, facilitating enhanced biological monitoring and ecological inference. The integration of environmental transcriptomics further expands our ability to assess ecosystem conditions, advancing biomonitoring approaches with unparalleled resolution. By delving into the mechanisms of eRNA production, degradation, and environmental persistence, we highlight key methodological advancements necessary for optimizing its application. We also address challenges and future directions, emphasizing the critical role of eRNA in bridging molecular ecology and conservation biology. As eRNA emerges as a next-generation tool for biodiversity surveillance, this chapter advocates for its broader implementation to revolutionize ecological research and environmental management.