In this paper recent experiments with the one–atom maser or micromaser are reviewed. They deal with the dynamical behavior of the field at parameter values where the field undergoes phase transitions. Furthermore, atomic interferences are observed in the micromaser when the inversion of the atoms leaving the cavity is measured while the cavity frequency is scanned across the atomic resonance. The interferences are due to the non–adiabatic mixing of dressed states at the entrance and exit holes of the maser cavity. The interference structures are triangular shaped and approximately equidistant. They are associated with the dynamics of the atom–field interaction, show quantum jumps and demonstrate bistability of the micromaser field.