This Chapter explores the conceptual evolution and interpretative challenges surrounding the Fair and Equitable Treatment (FET) principle in international investment law. Through a detailed case law analysis, the Chapter examines how arbitral tribunals have approached the persistent and problematic association of FET with the Minimum Standard of Treatment (MST) under customary international law (CIL). Based on this expansive case law analysis, it argues that this association has led to conceptual ambiguity, inconsistencies in arbitral decisions, and circular reasoning, undermining the clarity and predictability of the FET principle. The Chapter critiques the reliance on CIL/MST references, including the arbitrary invocation of precedents like the Neer case, and highlights the inadequacy of these references in clarifying the scope of FET. Instead, it notes that some new-generation IIAs increasingly codify specific acts and omissions that constitute violations of FET, reflecting a shift toward more concrete and coherent frameworks. While agreements such as CPTPP and RCEP maintain references to CIL, the EU’s practice diverges by articulating detailed obligations, detaching FET from its association with MST. Ultimately, the Chapter advocates for abandoning the MST/CIL linkage to achieve a clearer and more consistent application of FET. It argues that the growing body of arbitral decisions, despite their limitations, provides a foundation for defining specific sub-elements of FET, enabling the principle to evolve into a relatively consistent and transparent standard. This shift, as reflected in emerging treaty practices, signals a step toward a more reliable and fairness-oriented framework in investment law.