  In this paper we will present tha main features of what can be called
Schwinger's foundational approach to Quantum Mechanics. The basic ingredients
of this formulation are the \textit{selective measurements}, whose algebraic
composition rules define a mathematical structure called groupoid, which is
associated with any physical system. After the introduction of the basic axioms
of a groupoid, the concepts of observables and states, statistical
interpretation and evolution are derived. An example is finally introduced to
support the theoretical description of this approach.
