In every area of science there are some ideas that many students find difficult to grasp. A lack of understanding of key ideas can limit a student‟s ability to grasp and apply fundamental principles of their discipline. Previous work in this area by Taber has focused on this problem at school level. However little work has been done to systematically investigate and analyse this phenomenon in undergraduate science programmes beyond the anecdotal. A previous study involving students and staff from a range of scientific disciplines at our university identified that the mole (and its associated applications) was a difficult area for a wide range of students. The mole shows characteristics of being a „Threshold Concept‟ for students. Having identified the mole as a problem the aim here was to explore why it is difficult and whether the conceptual issues can be systematically overcome by using multiple perspectives. A sequence of questionnaires was used to survey over 100 people involving school students aged 14-17 first- second- and third-year university students and secondary school teachers in detail about the mole. We considered respondents‟ learning preferences and the type of activities in science lessons from which they felt they learned the most. Over 50% of respondents reported problems with the mole at some stage of their education. Further insights into how people conceptualise the mole were explored through the use of an educational research technique called "Hot Pen Writing". The outcomes of the research identified some of the reasons for student‟s lack of understanding of the mole concept including poor teaching in schools difficulty in relating the concept to real-life situations and not surprisingly the fact that it involves maths. We consider what this suggests about how and when the mole should be taught within school education.