Teachers’ and policy makers’ beliefs/ideologies are known to have considerable influ- ence on teaching practices (Cummins 2017). In Sweden’s current policy documents for English, monolingual teaching practices (‘English Only’) have strong ideological sup- port (cf. Skolverket 2011). A large-scale study (Skolinspektionen 2011) has shown that a bilingual, English-Swedish classroom language policy often emerges, however. With the current increase in students with a non-Swedish-speaking background, new beliefs and practices may emerge among teachers who teach English in language-diverse class- rooms, however. In such contexts, the English Only ideology may become even further entrenched since Swedish cannot be drawn on as a common background language (cf. Lundahl 2012). Alternatively, teachers may become influenced by the translanguaging ideology (García & Wei 2014), reconsider English Only and start drawing on multilin- gual students’ background languages as well as Swedish. In order to map in-service teachers’ beliefs and practices related to English Language Teaching (ELT) in language- diverse classrooms, a questionnaire was developed and administered online in 2017 to a random sample of English teachers in years 6-9 across Sweden. In this paper, we de- scribe how the questionnaire was designed, piloted and validated. Drawing on best prac- tice for the construction and use of questionnaires (Dörnyei & Taguchi 2010), we ac- count for the development of a pilot version, and discuss fundamental considerations in this process, such as construct definitions and operationalisations, validity and reliabil- ity, scale and item formats, and practical and theoretical challenges. The validation pro- cess included the use of raters for matching questions with targeted underlying con- structs, of an expert for advice on specific aspects, and an analysis of responses from a pilot administration of the instrument with teachers (N = 23) from the intended target population. The questionnaire will be made available to other researchers as a tool for capturing teacher beliefs about ELT in multilingual settings.