  The framework of generalized probabilistic theories is a powerful tool for
studying the foundations of quantum physics. It provides the basis for a
variety of recent findings that significantly improve our understanding of the
rich physical structure of quantum theory. This review paper tries to present
the framework and recent results to a broader readership in an accessible
manner. To achieve this, we follow a constructive approach. Starting from few
basic physically motivated assumptions we show how a given set of observations
can be manifested in an operational theory. Furthermore, we characterize
consistency conditions limiting the range of possible extensions. In this
framework classical and quantum theory appear as special cases, and the aim is
to understand what distinguishes quantum mechanics as the fundamental theory
realized in nature. It turns out non-classical features of single systems can
equivalently result from higher dimensional classical theories that have been
restricted. Entanglement and non-locality, however, are shown to be genuine
non-classical features.
