Travelogues in general include a broad variety of forms and contents, levels of credibility and intentions. They include diaries and letters, journal and narrative accounts, memoirs and essays, and record various types of experience of their authors. Travelogues have been popular reading since antiquity and remain popular today. Among their authors were missionaries and pilgrims, merchants and explorers, warriors and diplomats, engineers and physicians. The colonial expansion of the nineteenth century added a new purpose for travel and resulted in numerous accounts written by colonial officials. Because of their distinctiveness and variety, travelogues are best defined as a broad literary sub-category.