In this paper, we will provide an analysis of the first moment of Mozart’s String Quartet number 17, K 458, in Bb Major. This movement uses a classic sonata form and we will pay special attention to this aspect of the music. Through using a narrative form of analysis, that is analyzing the music in the order that it appears in the music, we will come to understand the different parts of sonata form. In particular, we will identify the exposition, the transition to the second key area, the second key area, the development and the recapitulation.