Few historical texts seem as familiar – or as compelling to read – as personal letters and diaries. They are plain-spoken, lively, and full of details. Both letters and diaries seem to emerge directly from the writer, fresh and intimate, bringing us close to who that person was. Both satisfy us by showing how people in the past shared many of our hopes, worries, and common sense. At the same time, both fascinate us by revealing differences between times past and our own time. They make us curious to explore differences in language and expressive styles, in what people felt needed saying and what did not. These differences in turn point to historical changes and continuities in self, social relations, work, and values, which personal letters and diaries capture with special sharpness.

Although diaries and letters from the past are immediate, homely, and thus comfortable to read, they are not as simple as they might first seem. And although diaries and letters are similar in important ways, each form has its own purposes and possibilities. Compared to many other kinds of written sources, both letters and diaries seem at first to be strikingly "private" kinds of writing. They give us the past from individual points of view. And yet, on closer look, almost any individual diary or letter resembles others from the same time and place. All were created and exchanged by classes of literate people who had the time and means to reflect and correspond. Consequently, in any given era, diaries and letters tend to follow certain shared forms or styles of what was considered to be appropriate or satisfying to express. Thus, although "private" in one sense, letters and diaries also may be seen as following certain widespread, "public" cultural conventions of expression (for example, diarists addressing their diaries as persons ) and topic (letter-writers talking about weather or health). For historians, then, it seems best to think of these writings as being personal rather private texts, inspiring us to look for commonalities among the individual examples.

The history of each form, especially since the seventeenth century, helps us sharpen a sense of how they are personal but not really private, and it helps us see how letters and diaries differ from each other as texts. The diary is a relatively recent form in the culture of western Europe and early America, arising in large part from a Christian desire to chart the story of individuals’ spiritual progress toward God. Such religious diaries broadened over time into the nineteenth-century practice of using diaries to record personal feelings and explore intellectual growth. Diaries thus were born of self-examination but expanded into a means of self-reflection and self-fashioning (experimenting with who one wants to be in the world). By the 1830s, diarists freely employed many of the literary devices of novels and other kinds of imaginative writing, especially writing by and for women. These aspects of diary-keeping continued into the twentieth century with an increasingly secular accent on psychological self-scrutiny and on using the diary as a means of emotional well-being and self-discovery.

In contrast, the letter, as a personal missive addressed to a particular recipient, is a much older form, dating from antiquity when ambassadors from one kingdom to another sent dispatches home, and, later, when travelers of various kinds reported on their journeys. Scholars have observed that many literary forms including official dispatches, newspapers, scientific studies, and even the modern novel arose from the letter’s particularity and sharp attention to place and character. (Even a diary may resemble a letter to oneself.) By the eighteenth century, European and American political and social commentators often framed their published remarks as "letters" to the public. This became a lively way to tell others about one’s interests or culture, and a flexible form for inscribing literate, bourgeois values in the education of youth, as publishers brought out instructive volumes of famous men’s letters and schools taught young American women and men proper ways to put into writing the relations of courtship, family, and business.

Overall, then, letters and diaries have certain points of difference as personal texts. Letters are written to a certain particular other; they implore a dialogue. Diaries are written for oneself or an imagined other; they play on the satisfactions of monologue. Letters are shaped by the contingencies of distance and time between writer and recipient; they become over time scattered in various places and must be "collected" to form a single body of writing. Diaries are shaped by moments of inspiration but also by habit; they are woven together by a single voice and usually are contained between covers. At the same time, letters and diaries share certain features. Diarists wrote letters and many letter-writers kept or read diaries. Their voices mingled and mixed. Both forms play with the tension between concealing and revealing, between "telling all" and speaking obliquely or keeping silent. Both inscribe the risks and pleasures of expression and trust. We will consider all of these further as we look at how historians use diaries and letters, keeping in mind as we use them that neither we nor people in past times know all there is to know. We need to collaborate.