This book represents the state of the art in the study of gradience in grammar - the degree to which utterances are acceptable or grammatical, and the relationship between acceptability and grammaticality. Among the questions considered in the book are: whether such measures are beyond the scope of a generative grammar or, in other words, whether the factors influencing acceptability are internal or external to grammar; whether observed gradience is a property of the mentally represented grammar or a reflection of variation among speakers; and what gradient phenomena reveal about the relationship between acceptability and grammaticality, and between competence and performance.