Mendelssohn’s correspondence, the most extensive of any composer up to his time, has always provided essential material for both his biography and study of the music. Fortunately, not only has a high proportion of probably around 6,500 letters survived – helped by their quality as well as the elegance of his script, which invited preservation – but they are importantly complemented by his own careful retention of incoming correspondence. The letters fall into three main groups: some 700 addressed to his family, regular communication with a small number of close friends, and those concerning his professional activities, including publication of his works. The story of the posthumous publication of his letters is traced, which culminated in the recent complete edition of them; this was only made possible thanks to many letters emerging from private ownership into publicly accessible archives since the Second World War.