  We describe how to obtain information on a quantum-mechanical system by
coupling it to a probe and detecting some property of the latter, using a model
introduced by von Neumann, which describes the interaction of the system proper
with the probe in a dynamical way.
  We first discuss single measurements, where the system proper is coupled to
one probe with arbitrary coupling strength. The goal is to obtain information
on the system detecting the probe position. We find the reduced density
operator of the system, and show how L\"uders rule emerges as the limiting case
of strong coupling.
  The von Neumann model is then generalized to two probes that interact
successively with the system proper. Now we find information on the system by
detecting the position-position and momentum-position correlations of the two
probes. The so-called "Wigner's formula" emerges in the strong-coupling limit,
while "Kirkwood's quasi-probability distribution" is found as the weak-coupling
limit of the above formalism. We show that successive measurements can be used
to develop a state-reconstruction scheme.
  Finally, we find a generalized transform of the state and the observables
based on the notion of successive measurements.

Quantum Physics