Models of $n\bar{n}$ transition in medium

The present state of the $n\bar{n}$ transition problem is briefly outlined. The models based on the diagram technique for direct reactions, potential description of $\bar{n}$-medium interaction and field-theoretical approach are considered. It is shown that for the $n\bar{n}$ transition in medium field-theoretical approach should be used. The lower limit on the free-space $n\bar{n}$ oscillation time $\tau_{{\rm min}}$ is found to be: $10^{16}\; {\rm yr}>\tau_{{\rm min}}>1.2\cdot 10^{9}\; {\rm s}$.