Existing research suggests that the force and flow of globalisation and the consequent use of English as the global lingua franca has a significant impact on people's identity formation. Many postcolonial countries use English as a second or foreign language and thus have adopted it as the language for instruction in schools. The use of English as the medium of instruction (EMI) has been widely studied and it has been found that this has an impact on people's identity formation. Although extensive research on EMI and identity formation has been conducted, this remains an under-researched area in many multilingual contexts, for example, in the Punjab in India. Moreover, research that does exist has not focused on high school students' views and experiences. To begin to address this gap in the existing published literature, this qualitative study set out to investigate whether, and to what extent, the language used as the MOI in Punjab in India affects high school students' perceptions of their identity formation. Two schools were involved: one where English was used as the medium of instruction, and one where Punjabi was used as the medium of instruction. Data were gathered using documentary analysis; four focus group discussions with pupils (two in each of the two schools, involving a total of twenty four students); and semi-structured interviews with two headteachers, four English language teachers (two from each school) and twelve high school students in 9th and 10th grades (six from each school). Participants' views and perceptions concerning the language of instruction and its impact on students' opportunities to access higher education programmes, their future careers, and their social positions in Punjabi society were sought. The conceptual framing of the study draws on Norton's concept of investment (Norton, 1995; Norton, 2016b) which includes ideology, agency and identity. In addition, significant concepts such as Bourdieu's (1986) forms of capital and Markus and Nurius's (1986) notion of 'possible selves' have also been used. The data were analysed using Charmaz's (1995, 2006, 2014) account of constructivist grounded theory, and a constant comparative analysis approach was employed to identify similarities and differences in participants' accounts and to locate dominant themes within and across the data sets. Key findings revealed that the use of English as the medium of instruction was seen by each participant group to have a significant positive impact on students' identity formation and of their perceptions of their future possible selves; on their opportunities to embark on higher education degrees, particularly in areas such as medicine and science; on their future employment opportunities; and on their social positions in Punjab. This research contributes geographically, theoretically, as well as methodologically to the research in the area of EMI and identity and may help to raise awareness about learning and teaching in different languages and its impact on students' identity formation. This study concludes with a discussion of the implications of findings from the study for policymakers in Punjab, for language teachers, for the education and training of teachers, for parents, and above all for the students.