Spectral theory is a crucial aspect of Functional Analysis. In applications many eigenvalue problems arise, it is therefore expedient to study the criteria that guarantees the existence of basic system for linear operators. This work contains an in-depth study of the compact linear operators and their spectrum. Complexification of real Banach spaces is treated to allow the immediate extension of many results on real Banach spaces and their operators to Complex Banach space.It is interesting to note that compact alone does not guarantee the existence of basic system. Spectral decomposition theory for compact and selfadjoint operators is used to justify the existence of basic system for unbounded operators(non-compact operators). This work is not useful to the Pure Mathematicians alone but also to the Applied Mathematicians,Physicist and Engineers because it also contains applications to elliptic boundary value problems and other optimization problems.