This paper investigates the role of resource allocation as a source of processing difficulty in human sentence comprehension. It proposes a simple information-theoretic characterization of processing difficulty as the work incurred by resource reallocation during parallel, incremental, probabilistic disambiguation in sentence comprehension, and demonstrates its equivalence to the theory of Hale (2001), in which the difficulty of a word is proportional to its surprisal (its negative log-probability) in the context within which it appears. This proposal subsumes and clarifies findings that high-constraint contexts can facilitate lexical processing, and connects these findings to well-known models of parallel constraint-based comprehension. In addition, the theory leads to a number of specific predictions about the role of expectation in syntactic comprehension, including the reversal of locality-based difficulty patterns in syntactically constrained contexts, and conditions under which increased ambiguity facilitates processing. The paper examines a range of established results bearing on these predictions, and shows that they are largely consistent with the surprisal theory. The paper argues that the surprisal theory, when conjoined with probabilistic models chosen according to appropriate principles, makes a wide range of precise predictions consistent with empirical observations, while remaining relatively neutral as to the exact representations of possible structural analyses. Section 4 contrasts the surprisal theory with alternative resource-allocation and resource-limitation theories of processing difficulty, illustrating the general conditions under which their predictions maximally diverge. The remainder of the paper examines a number of established experimental results pertaining to these divergent predictions, and shows that they lend considerable support to the surprisal theory.This paper investigates the role of resource allocation as a source of processing difficulty in human sentence comprehension. It proposes a simple information-theoretic characterization of processing difficulty as the work incurred by resource reallocation during parallel, incremental, probabilistic disambiguation in sentence comprehension, and demonstrates its equivalence to the theory of Hale (2001), in which the difficulty of a word is proportional to its surprisal (its negative log-probability) in the context within which it appears. This proposal subsumes and clarifies findings that high-constraint contexts can facilitate lexical processing, and connects these findings to well-known models of parallel constraint-based comprehension. In addition, the theory leads to a number of specific predictions about the role of expectation in syntactic comprehension, including the reversal of locality-based difficulty patterns in syntactically constrained contexts, and conditions under which increased ambiguity facilitates processing. The paper examines a range of established results bearing on these predictions, and shows that they are largely consistent with the surprisal theory. The paper argues that the surprisal theory, when conjoined with probabilistic models chosen according to appropriate principles, makes a wide range of precise predictions consistent with empirical observations, while remaining relatively neutral as to the exact representations of possible structural analyses. Section 4 contrasts the surprisal theory with alternative resource-allocation and resource-limitation theories of processing difficulty, illustrating the general conditions under which their predictions maximally diverge. The remainder of the paper examines a number of established experimental results pertaining to these divergent predictions, and shows that they lend considerable support to the surprisal theory.