  We summarize our study of the neutron-antineutron transition in medium and
nuclei in the framework of the field-theoretical approach. The models based on
the diagram technique for direct reactions, potential description of
antineutron-medium interaction are considered as well. The lower limit on the
free-space neutron-antineutron oscillation time $\tau $ is in the range:
$10^{16}\; {\rm yr}>\tau >1.2\cdot 10^{9}\; {\rm s}$.
