  We extend the theory of perturbations of KMS states to some class of
unbounded perturbations using noncommutative Lp-spaces. We also prove certain
stability of the domain of the Modular Operator associated to a
||.||p-continuous state. This allows us to define an analytic multiple-time KMS
condition and to obtain its analyticity together with some bounds to its norm.
  Apart from that, this work contains a detailed review, with minor
contributions due to the author, starting with the description of C*-algebras
and von Neumann algebras followed by weights and representations, a whole
chapter is devoted to the study of KMS states and its physical interpretation
as the states of thermal equilibrium, then the Tomita-Takesaki Modular Theory
is presented, furthermore, we study analytical properties of the modular
operator automorphism group, positive cones and bounded perturbations of
states, and finally we start presenting multiple versions of noncommutative
Lp-spaces.
