Half a century after Arthur Conan Doyle wrote the first story, Sherlock Holmes still exerted a considerable grip on the public imagination, attributable not just to his immediately identifiable iconography but to the particular values for which he stood; values which could be summarised as ‘certainty in a time of uncertainty’. This concept of certainty is integral to the classic detective genre, in which comfort and reassurance are key elements. This chapter explores these and other elements of the genre, establishing its parameters and how it differs from allied genres such as the thriller. Whereas later chapters will look in detail at the Holmes films themselves for generic ‘clues’, the aim here is to examine expectations: what contemporary audiences expected from a detective film; how, and why, this differed from a detective novel; and how these expectations tallied with the way Twentieth Century-Fox and Universal promoted the Holmes films. The latter point is especially relevant given Universal’s ‘modernising’ of the character and the straying of the series across genre boundaries. One motivation for this is that the studio was endeavouring to compete with, or counter, the emergence of the hardboiled detective genre. As ‘certainty’ was an early casualty in the hardboiled film, this is a particularly interesting development.