This chapter describes Alfred Tarski's work as a secondary-school mathematics teacher and teacher-trainer in Poland during 1924–1935. It also provides background for that activity, and for all of Tarski's publications that stemmed from it. The first of those is translated in chapter 10: his [1929] 2014a report to secondary teachers about the First Congress of Mathematicians of Slavic Countries, held in 1929 in Warsaw.