Acidic pH is a well-established hallmark of the tumor microenvironment, arising from dysregulated tumor metabolism and inadequate vascular perfusion. This acidification not only supports tumor progression and metastasis but also serves as a promising target for tumor detection and surgical navigation. This chapter provides a comprehensive overview of pH sensing strategies, with a particular focus on fluorescent probes designed to detect extracellular and intracellular pH alterations in tumor and cellular contexts. Various types of fluorescent probes—including those based on photoinduced electron transfer (PeT), intramolecular charge transfer (ICT), Förster resonance energy transfer (FRET), aggregation-caused quenching (ACQ), and aggregation-induced emission (AIE)—are described in detail, highlighting their mechanisms, pH response ranges, and applications in live-cell imaging and in vivo tumor diagnostics. Furthermore, the development of dual-responsive probes, which respond to both pH changes and tumor-associated biomarkers such as matrix metalloproteases, is presented as a strategy to improve probe specificity and reduce false positives in complex biological environments. Collectively, this chapter underscores the critical role of pH-responsive fluorescence imaging tools in cancer biology and their potential to enhance precision diagnostics and therapeutic interventions.