Children experience numerous types of learning transitions throughout their educational lives physically (changing schools) and substantively (moving through different key stages in school), and daily (in moving between subjects). One transition that is frequently overlooked within the British education system is that from the General Certificate of Secondary Education (GCSE) usually completed at the age of 16 to Advanced Subsidiary Level (AS Level) usually completed at the age of 17. This case study addresses this issue by focusing on students' perceptions and experiences of the transition between GCSE and AS Level mathematics. It involves a small qualitative study of 28 students from a post-16 college in the UK and uses questionnaires and semi-structured interviews to ascertain students' views of their transition in studying mathematics at a higher level. The evidence suggests that many students struggle with the transition from GCSE to A Level making AS Level Mathematics inaccessible for many students. Ways of addressing the difficulties experienced by students are discussed with implications for teachers, students and policy-makers in the hope of supporting smoother learning transitions to advanced level study for all.