The Secondary Analysis of TIMSS (Third International Mathematics and Science Study) Video Data study used TIMSS Videotape Classroom Study data and vPrism software to achieve methodological and substantive findings on teacher quality, instructional practices, and classroom interactions. It discussed methodological and technical issues arising during secondary analysis of existing data sets using specific software applications. The paper examines three methodological findings from using the video data and software to explore teachable moments: the technical dictates of how an utterance is defined and operationalized for coding may impact a study's unit of analysis and overall design; coders coding in pairs or groups may be more reliable; and it is important to do the empirical work of developing a study design, codes, and coding and analysis protocols and to critically reflect on assumptions and processes involved. This secondary analysis resulted in increased awareness of the significant interplay between the unit of analysis, the coding unit, and the software application. The study showed that it was fruitful to use the TIMSS video data set for secondary analysis. There appeared to be an instructional practice called a "teachable moment" as defined, and it was possible to develop, apply, and test codes and their reliability. (SM)