Realism and Naturalism are two closely related literary movements that emerged in the 19th century, with Realism focusing on depicting everyday life and society as it is, while Naturalism takes a more scientific approach, emphasizing the influence of environment and heredity on human behavior. Both movements aimed to present a truthful portrayal of life, but Naturalism often delves deeper into the deterministic forces that shape characters' fates, making it more pessimistic in tone compared to Realism.