 Although semi-structured interviews, SSIs, are used extensively in research, scan attention is given to their diversity, underlying assumptions, construction, and broad applications to qualitative and mixed method research. In this three-part article, we discuss the following, A, how the SSI is situated historically including its evolution and diversification, B, the principles of constructing SSIs, and, C, how SSIs are utilized as a standalone research method and a strategy within a mixed method design. This article was authored by Michelle J. McIntosh and Janice M. Morse.