A summary of lectures is presented to give an account of a particular approach to quantum field theory. This approach is being incorporated in some areas as an "approach from basic principles." The principles discussed, which lead to the systematic analysis approach, are: the basic mathematics of quantum physics; the invariance of the theory under inhomogeneous Lorentz transformations; and the fact that all the states of the system described correspond to collision processes between particles, the number of which may be arbitrarily large but never infinite. This third postulate serves to select that particular type of representation of the inhomogeneous Lorentz group which is of interest in the quantum field theory. By assuming that the basic quantities in which all previously defined notions have to be expressed are operator fields with a certain set of properties, the connection between the described S-matrix frame and the quantum field theory is established.