This chapter describes the factors influencing speech intelligibility, which include human factors such as articulation and technical factors such as reverberation. Various measures of intelligibility are described, with a focus on the speech intelligibility index (STI), which is both predictable and measurable and has an established relationship with word recognition. Optimising acoustic performance for speech intelligibility is discussed, with a summary of good design practice. A spreadsheet is provided to estimate STI in simple cases.