Vol. 31 Num. 2 (2011) Richard Barwell Editorial2 Colin Foster Productive ambiguity in the learning of mathematics ABSTRACT 3 Rina Zazkis , Ami Mamolo Reconceptualizing knowledge at the mathematical horizon ABSTRACT 8 Laurinda Brown Communication: What is a concept?14 David Reid Communication: An enactivist reaction to Samson & Schäfer16 David Tall Communication: Looking for the bigger picture17 Walter Whiteley Communication: Modeling for life18 Uwe Gellert Communication: Now it concerns us! A reaction to Sustainable Mathematics Education19 Robert Ward-Penny , Sue Johnston-Wilder , Clare Lee Exit interviews: undergraduates who leave mathematics behind ABSTRACT 21 Indigo Esmonde Snips and snails and puppy dogs' tails: genderism and mathematics education ABSTRACT 27 Michal Tabach Communication: The dual role of researcher and teacher: a case study32 Andrea Rohrer , Gert Schubring Ethnomathematics in the 1930s - the contribution of Ewald Fettweis to the history of ethnomathematics ABSTRACT 35 - Contributor affiliations and contact information40