We prove that it is decidable whether a finitely based permutation class contains infinitely many simple permutations, and establish an unavoidable substructure result for simple permutations: every sufficiently long simple permutation contains an alternation or oscillation of length k. Key words: permutation class, restricted permutation, simple permutation 1991 MSC: 03B25, 03D05, 05A15, 68F10, 68Q45