This issue of Research in Mathematics Education (RME) marks a special point in the history of the journal. As incoming editors (beginning with volume 20), it is our honour to introduce volume 21 to celebrate 21 years of RME. As noted by the editors of volume 10 (Rowland & Nardi, 2008a), RME, an international journal with the explicit aim of being informative and relevant to researchers in the field worldwide , is the official journal of the British Society for Research into Learning Mathematics (BSRLM). Volume 10 coincided with the thirtieth anniversary of the formation of the British Society for the Psychology of Learning Mathematics (BSPLM), renamed BSRLM in 1985. The BSRLM spirit is at the heart of RME. Indeed, the editor of volume 1 (Brown, 1999, p. ii) of the journal wrote about being "excited by the ideas represented [in the first issue] and therefore discussed within the community of researchers and teachers which is BSRLM". Likewise, the editors of volume 3 (Jones & Morgan, 2001, p. 3) drew attention to how the articles display interest in "a broad range of issues in mathematics education, making use of different theoretical frameworks and methodologies and including both reports of empirical studies and more theoretical contributions". The early editions of the journal were published as single volumes (in effect, double issues). This changed with volume 10 when a pattern of two issues per year commenced. Two years later, in 2010, the journal editors (Nardi & Rowland, 2010) announced the journal's growth to three issues per year, beginning with Volume 13. This established a pattern whereby Issues 1 and 3 of each volume are 'regular' issues and Issue 2 is a 'Special Issue' (SI) with guest editors. The first SI was entitled Deepening engagement in mathematics in pre-university education, with its introductory article (Wake, 2011) explaining how the SI reported on the outcomes of the TLRP project, a major funded UK research project (see: www.transmaths.org). Subsequent Special Issues, to date, have been entitled European research in mathematics education