This review discusses the importance of acidity in cancer cells and its impact on therapy. Cancer cells often exhibit high glucose metabolism, leading to acidic tumor environments. This acidity can promote tumor aggressiveness, invasiveness, and metastasis. The review explores the origins of tumor acidity, mechanisms of resistance, and challenges in measuring intracellular pH. It highlights the role of acidity in tumor development and its dual role in therapy. The review also discusses the regulation of intracellular acidity through proteins like NHEs, V-ATPase, MCTs, and carbonic anhydrases. These proteins help maintain intracellular pH and manage lactate and H+ ions. The review further examines the consequences of acidity on tumor invasion, cellular plasticity, immunosuppression, and the Warburg effect. It also addresses the evolution of methods for measuring intracellular pH, including pH microelectrodes, 31P NMR spectroscopy, weak acids and bases, and fluorescence microscopy. Finally, the review discusses the role of acidity in therapy, including strategies to target tumor acidity and improve drug delivery and efficacy.This review discusses the importance of acidity in cancer cells and its impact on therapy. Cancer cells often exhibit high glucose metabolism, leading to acidic tumor environments. This acidity can promote tumor aggressiveness, invasiveness, and metastasis. The review explores the origins of tumor acidity, mechanisms of resistance, and challenges in measuring intracellular pH. It highlights the role of acidity in tumor development and its dual role in therapy. The review also discusses the regulation of intracellular acidity through proteins like NHEs, V-ATPase, MCTs, and carbonic anhydrases. These proteins help maintain intracellular pH and manage lactate and H+ ions. The review further examines the consequences of acidity on tumor invasion, cellular plasticity, immunosuppression, and the Warburg effect. It also addresses the evolution of methods for measuring intracellular pH, including pH microelectrodes, 31P NMR spectroscopy, weak acids and bases, and fluorescence microscopy. Finally, the review discusses the role of acidity in therapy, including strategies to target tumor acidity and improve drug delivery and efficacy.