Past attempts to relate two well-known models of backtracking computation have met with only limited success. We relate these two models using logical relations. We accommodate higher-order values and infinite computations. We also provide an operational semantics, and we prove it adequate for both models. Categories and Subject Descriptors D.3.1 [Programming Languages]: Formal Definitions and Theory; F.3.2 [Logics and Meanings of Programs]: Semantics of Programming Languages--Denotational seman