A historian is a person who studies and writes about the past and is regarded as an authority on it. Historians are concerned with the continuous, methodical narrative and research of past events as relating to the human race; as well as the study of all history in time. Some historians are recognized by publications or training and experience. "Historian" became a professional occupation in the late nineteenth century as research universities were emerging in Germany and elsewhere.

In the 19th century, scholars used to study ancient Greek and Roman historians to see how generally reliable they were. In recent decades, however, scholars have focused more on the constructions, genres, and meanings that ancient historians sought to convey to their audiences. History is always written with contemporary concerns and ancient historians wrote their histories in response to the needs of their times. Out of thousands of Greek and Roman historians, only the tiniest fraction's works survive and it is out of this small pool that ancient historians and ancient historiography are analyzed today. Modern historians of the ancient world have to deal with diverse types of evidence, which are debated more today than in the 19th century due to innovations in the field.

Ancient historians were very different from modern historians in terms of goals, documentation, sources, and methods. For instance, chronological systems were not widely used, their sources were often absorbed (traceability of such sources usually disappeared), and the goal of an ancient work was often to create political or military paradigms. It was only after the emergence of Christianity that philosophies of history grew in prominence due to the destiny of man from the Christian account. Epics such as Homer's works were used by historians and considered history even by Thucydides. Ancient orators like Cicero stated that their historiographical narrative was divded into 3 divisions: history, argument, and fable.

In the 19th-century historical studies became professionalized at universities and research centers along with a belief that history was a type of science. However, in the 20th century historians incorporated social science dimensions like politics, economy, and culture in their historiography, including postmodernism. Since the 1980s there has been a special interest in the memories and commemoration of past events.

History by its nature is prone to continuous debate, and historians tend to be divided. There is no past that is commonly agreed upon, since there are competing histories (e.g., of elites, non-elites, men, women, races, etc.). It is widely accepted that "strict objectivity is epistemologically unattainable for historians". Historians rarely articulate their conception of objectivity or discuss it in detail. And like in other professions, historians rarely analyze themselves or their activity. In practice, "specific canons of historical proof are neither widely observed nor generally agreed upon" among professional historians. Though objectivity is often seen as the goal of those who work on history, in practice there is no convergence on anything in particular. Historical scholarship is never value free since historian's writings are impacted by the frameworks of their times. Some scholars of history have observed that there are no particular standards for historical fields such as religion, art, science, democracy, and social justice as these are by their nature 'essentially contested' fields, such that they require diverse tools particular to each field beforehand in order to interpret topics from those fields.

There are three commonly held reasons why avoiding bias is not seen as possible in historical practice: a historian's interest inevitably influences their judgement (what information to use and omit, how to present the information, etc.); the sources used by historians for their history all have bias, and historians are products of their culture, concepts, and beliefs. Racial and cultural biases can play major roles in national histories, which often ignore or downplay the roles on other groups. Gender biases as well. Moral or worldview evaluations by historians are also seen partly inevitable, causing complications for historians and their historical writings. One way to deal with this is for historians to state their biases explicitly for their readers. In the modern era, newspapers (which have a bias of their own) impacts historical accounts made by historians. Wikipedia also contributes to difficulties for historians.

During the Irving v Penguin Books and Lipstadt trial, the court relied on Richard Evan's witness report which mentioned "objective historian" in the same vein as the reasonable person, and reminiscent of the standard traditionally used in English law of "the man on the Clapham omnibus". This was necessary so that there would be a legal benchmark to compare and contrast the scholarship of an objective historian against the illegitimate methods employed by David Irving, as before the Irving v Penguin Books and Lipstadt trial, there was no legal precedent for what constituted an objective historian.

Justice Gray leant heavily on the research of one of the expert witnesses, Richard J. Evans, who compared illegitimate distortion of the historical record practiced by Holocaust deniers with established historical methodologies.