Suppose a string $X_{1}^{n}=(X_{1},X_{2},ldots ,X_{n})$ generated by a memoryless source $(X_{n})_{ngeq 1}$ with distribution $P$ is to be compressed with distortion no greater than $Dgeq 0$, using a memoryless random codebook with distribution $Q$. The compression performance is determined by the “generalized asymptotic equipartition property” (AEP), which states that the probability of finding a $D$-close match between $X_{1}^{n}$ and any given codeword $Y_{1}^{n}$, is approximately $2^{-n R(P,Q,D)}$, where the rate function $R(P,Q,D)$ can be expressed as an infimum of relative entropies. The main purpose here is to remove various restrictive assumptions on the validity of this result that have appeared in the recent literature. Necessary and sufficient conditions for the generalized AEP are provided in the general setting of abstract alphabets and unbounded distortion measures. All possible distortion levels $Dgeq 0$ are considered; the source $(X_{n})_{ngeq 1}$ can be stationary and ergodic; and the codebook distribution can have memory. Moreover, the behavior of the matching proba-bility is precisely characterized, even when the generalized AEP is not valid. Natural characterizations of the rate function $R(P,Q,D)$ are established under equally general conditions.
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