The papers in this collection, drawn from the 34th Annual Conference of the British Association for Applied Linguistics, reflect a number of different perspectives within the field of applied linguistics at the start of the twenty-first century. While addressing the theme of unity and diversity, each paper prompts critical reflection on tensions within the discipline between stability and change, consensus and controversy, similarity and variation. The interpretation of language use is broad and varied, taking both macro- and micro-perspectives. Topics addressed range from issues of global communication in a world of shifting demographies and technological advances to analyses of specific contexts of interaction, both professional and personal. Contexts of language use frequently coincide with settings of language acquisition, both within and beyond the language classroom, and this opens up discussion of the focus, scope and appropriateness of research stances in applied linguistics and practices in language pedagogy. Futhermore, variation is considered from a number of social-cultural, gender-related, linguistic and discourse perspectives, calling into question terminology, definitions and the nature of evidence at the heart of applied linguistic theory and practice.