This monograph presents Unified Substrate Theory (UST): a necessity–based framework for identifying the minimal structural conditions underwhich physical description is possible. UST proceeds by suspending all ontological commitments—particles, fields, spacetime, quantum states, or fundamental interactions—and instead asks a prior question: what structural constraints must hold for any physical theory to be formulable, testable, and stable under admissible perturbations?The theory advances no microscopic model and proposes no new fundamen- tal entity. Its method is structural and eliminative.The work is divided into three logically closed layers. The first establishes the admissibility protocol and derives discrete logarithmic scalestructure, a unique dominant spectral band, emergent geometry, emergent time as a spectral ordering observable, the necessity of matter as diagnos-tic carriers, gravity as a conservative low–energy closure, and cosmology as a non–fundamental fixed–point regime. The second supplies the minimaldynamical content consistent with these constraints, formulating regeneration dynamics in symbol–first form and proving a no–plurality theoremfor persistent logarithmic bands under admissible perturbations, noise, and coarse–graining. The third presents a closed set of empirical tribunals, eachsatisfying strict pre–declaration, null calibration, scan correction, and outcome reporting criteria. UST is intentionally fragile. Its claims are not adjustable and admit explicit conditions of structural breakdown. Empirical data serve solely as atribunal of applicability; agreement does not confirm the framework, while violation of its structural necessities terminates it. Established theories—classical mechanics, quantum theory, general relativity, and ΛCDM cosmology— are neither reduced nor replaced, but situated as valid regime–level descrip-tions within explicitly delimited domains. UST asserts no ontology and no finality. Its sole aim is to delineate which elements of physical description are enforced by necessity, which areconditional, and which exceed the limits of admissible explanation