  Astrophysical sources are now observed by many different instruments at
different wavelengths, from radio to high-energy gamma-rays, with an
unprecedented quality. Putting all these data together to form a coherent view,
however, is a very difficult task. Each instrument has its own data format,
software and analysis procedure, which are difficult to combine. It is for
example very challenging to perform a broadband fit of the energy spectrum of
the source. The Multi-Mission Maximum Likelihood framework (3ML) aims to solve
this issue, providing a common framework which allows for a coherent modeling
of sources using all the available data, independent of their origin. At the
same time, thanks to its architecture based on plug-ins, 3ML uses the existing
official software of each instrument for the corresponding data in a way which
is transparent to the user. 3ML is based on the likelihood formalism, in which
a model summarizing our knowledge about a particular region of the sky is
convolved with the instrument response and compared to the corresponding data.
The user can choose between a frequentist analysis, and a Bayesian analysis. In
the former, parameters of the model are optimized in order to obtain the best
match to the data (i.e., the maximum of the likelihood). In the latter, the
priors specified by the user are used to build the posterior distribution,
which is then sampled with Markov Chain Monte Carlo or Multinest. Our
implementation of this idea is very flexible, allowing the study of point
sources as well as extended sources with arbitrary spectra. We will review the
problem we aim to solve, the 3ML concepts and its innovative potential.
