The theory and practice of assistance to speech communities whose native languages are threatened are examined. The discussion focuses on why most efforts to reverse language shift are unsuccessful or even harmful, diagnosing difficulties and prescribing alternatives based on a combination of ethnolinguistic, sociocultural, and econotechnical considerations. The first section looks at reasons for trying to reverse language shift, whether it is possible, where and why shift occurs, and how it can be reversed. A typology of disadvantaged languages and ameliorative priorities is presented. The second section contains a number of case studies from different continents, including those of: Irish; Basque; Frisian; Navajo, Spanish, and Yiddish (secular and ultra-orthodox) in America; Maori in New Zealand; Australian aboriginal and immigrant languages, Modern Hebrew and French in Quebec; Catalan in Spain; and the unique situation of non-transmission of "additional" (parents' second) languages. The third section addresses the related issues of language planning for reversal of language shift, dialect standards and corpus planning, intergenerational transmission of "additional" languages, and limitations on school effectiveness in connection with native language transmission. Chapters include bibliographies. (MSE)