This paper explores the deep connections between spectral theory and algebraic topology, focusing on the decomposition of operator spectra based on underlying topological invariants. In quantum condensed matter physics and mathematics, the structure of an operator's spectrum provides fundamental insights into the system's properties. When the system possesses a nontrivial topology, for example in topological insulators or quantum Hall systems, the spectrum exhibits protected features, such as gapless edge states or quantized conductances, which are robust against continuous deformations. We formalize the concept of a topological spectral decomposition by leveraging tools from K-theory, C*-algebras, and non-commutative geometry. This framework allows for a classification of spectral gaps and in-gap states according to topological indices, such as Chern numbers or Z2 invariants. The core of this work is to establish a rigorous bulk-boundary correspondence, demonstrating that the decomposition of the bulk spectrum into topologically distinct regions directly predicts the number and nature of states localized at the system's boundary. We review key theoretical models, including the Su-Schrieffer-Heeger (SSH) model and integer quantum Hall effect, to illustrate how topological invariants partition the spectrum and dictate the physical observables. The methodology presented provides a unified mathematical structure for understanding and predicting the spectral properties of a wide range of topological materials, offering a powerful tool for both theoretical analysis and the design of new quantum materials.