  The usual formulation of thermodynamics is based on the additivity of
macroscopic systems. However, there are numerous examples of macroscopic
systems that are not additive, due to the long-range character of the
interaction among the constituents. We present here an approach in which
nonadditive systems can be described within a purely thermodynamics formalism.
The basic concept is to consider a large ensemble of replicas of the system
where the standard formulation of thermodynamics can be naturally applied and
the properties of a single system can be consequently inferred. After
presenting the approach, we show its implementation in systems where the
interaction decays as $1/r^\alpha$ in the interparticle distance $r$, with
$\alpha$ smaller than the embedding dimension $d$, and in the Thirring model
for gravitational systems.
