Differentiation is a philosophy of education which recognizes that pupils learn differently. This chapter addresses the what, why and how questions which face teachers across the curriculum as they seek to embed differentiated teaching into their practice. The need to ensure that each pupil experiences meaningful and successful learning often seems a daunting challenge given material and time constraints, but differentiation is manageable when viewed as flexibility in planning, teaching and assessing. The range of strategies which constitute differentiation also underpin recent conceptual innovations in education such as 'personalised learning' (DfES 2004) and 'assessment for learning' (Black et al. 2003), sharing the aims of empowering pupils through developing the learning skills which work best for them. There is no great mystery to differentiation, yet it often appears to be an elusive concept. I asked 50 training teachers nearing the end of their PGCE year to report on their experiences of differentiation and most had not heard of the term before starting the course, unless they had encountered it in pre-course reading. Almost all agreed with the principles of differentiation but still felt that they were unsure how to implement these in their lessons. Similarly, subject mentors often report that training teachers do not adequately consider individual learners' needs. Yet, differentiation is not an 'extra' dimension to teaching, rather, it represents a set of principles and practices which are teaching in the modern classroom.