Diaries are rich but sometimes challenging sources for historians, not least because of their particularity, which can make it difficult to generalize from them. This chapter outlines some of the ways scholars have approached diaries, highlighting the comparative method used by historians such as James Hinton. The relationship between, in Fothergill’s words, ‘the first-person narrator who speaks in the diary and the historical personage who held the pen’ is considered and Huff’s view that we should read diaries as ‘friendly explorers’ endorsed. Questions relating to when, why and for whom a diary may have been written are discussed and the equally important issue of what a diary omits or suppresses. The exceptional potential of long-run, unpublished diaries as source material (as used here) is underlined. Finally this chapter explains the principles on which the diaries on which the book is based were selected and the extent to which they may or may not be representative.