  Observables and instruments have played significant roles in recent studies
on the foundations of quantum mechanics. Sequential products of effects and
conditioned observables have also been introduced. After an introduction in
Section~1, we review these concepts in Section~2. Moreover, it is shown how
these ideas can be unified within the framework of measurement models. In
Section~3, we illustrate these concepts and their relationships for the simple
example of a qubit Hilbert space. Conditioned observables and their
distributions are studied in Section~4. Section~5 considers joint probabilities
of observables. We introduce a definition for joint probabilities and discuss
why we consider this to be superior to the standard definition.

Quantum Physics