  Allpix$^2$ is a versatile, open-source simulation framework for silicon pixel
detectors. Its goal is to ease the implementation of detailed simulations for
both single sensors and more complex setups with multiple detectors. While
originally created for silicon detectors in high-energy physics, it is capable
of simulating a wide range of detector types for various application scenarios,
through its interface to Geant4 to describe the interaction of particles with
matter, and the different algorithms for charge transport and digitization. The
simulation chain is arranged with the help of intuitive configuration files and
an extensible system of modules, which implement the individual simulation
steps. Detailed electric field maps imported from TCAD simulations can be used
to precisely model the drift behavior of the charge carriers, bringing a new
level of realism to the Monte Carlo simulation of particle detectors. Recently,
Allpix$^2$ has seen major improvements to its core framework to take full
advantage of multi- and many-core processor architectures for simulating events
fully parallel. Furthermore, new physics models such as charge carrier
recombination in silicon have been introduced, further extending the
application range. This contribution provides an overview of the framework and
its components, highlighting the versatility and recent developments.
