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There is no generally accepted definition of conditional Rényi entropy. The (unconditional) Rényi entropy depends on a parameter α, which for the case of min-entropy takes the value ∞. Even for this particular case, there are several proposals for the definition of conditional entropy. This paper describes three general definitions of conditional Rényi entropy that were found or suggested in the literature. Their properties are studied and their values, as a function of α, are compared. The particular case of min-entropy is widely used in cryptography as a security parameter; this case is studied in some detail.