The paper considers several different stability notions for discrete-time linear time-varying systems specified by almost-periodic kernels. Various input-output stability notions are analysed, including those in which the signal spaces consist of bounded power and asymptotically almost-periodic functions. Further, it is proved that every stable time-varying discrete time system commuting with a polynomial in the shift operator is periodic. An example shows that this result does not hold for unstable time-varying systems. (C) 2000 Academic Press.