This paper examines the process through which three pre-service teachers learn to use mathematical tools; it also looks at pre-service teachers' instrumentation of tools into mathematics teaching. Three pre-service teachers were studying at a primary mathematics teacher training program at Dokuz Eylul University in Turkey. During an eight-week period, workshops were conducted on curriculum tools with pre-service teachers. Subsequently, pre-service teachers' lessons were observed in real school settings. The findings are underpinned by the theoretical framework based on the instrumental approach to tool use. Results indicate that pre-service teachers had difficulty in applying the appropriate use of tools, and teachers' instrumentation schemes influenced students' conceptual understanding. The analysis of data also suggests some implications about the usefulness of the instrumentation framework for effective integration of tools into mathematics teaching. (Contains 1 table and 6 figures.)