Condensation is a widespread phenomenon that appears in various physical systems. From a mathematical point of view it can be seen as the spontaneous migration of a macroscopic number of particles to some region of the space. As a simple model I discuss the condensation phenomenon for a stochastic particle system known as Zero Range Process. In this system there is a one to one correspondence between condensates and metastable configurations. After a suitable time scaling, stochastic tunneling occurs and the system evolves among condensates.