Complexity science, which arose in the second half of the 20th century, initiated research into the emergence of complex systems and led to the rise of the concept of diachronic emergence. Compared to British emergentism, research on diachronic emergence underwent some crucial changes—namely, (1) putting the enterprise of unveiling the mechanics of emergence at its core; (2) taking inter-disciplinary research as its viewpoint; (3) and taking computer simulation as its method. Because of this new approach, “diachronic emergence” is closely related to terms from complexity science such as “systems,” “self-organization,” “complexity,” and “chaos.” In this paper, we examine two cases of purported diachronic emergence and argue that both count as genuine cases of ontological emergence. The first is Paul Humphreys’ fusion emergence and the second is Mark Bedau’s simulation emergence. In both cases, the emergent entity/property possesses genuine causal powers, and hence counts as a form of ontological, not merely epistemological emergence. Fusion emergence is a kind of strong diachronic emergence that emphasizes diachronicity and non-supervenience. The kind of emergence based on computer simulations can be seen as a kind of weak diachronic emergence. Bedau studies the process and mechanics of emergence with the help of computer simulations, and he argues that weak diachronic emergence has characteristics such as underivability without simulation, explanatory incompressibility, and underivability without crawling the micro-causal web. Moreover, he tries to present an explanatory model of weak emergence that posits the existence of higher-level entities with weak downward causation and claims the emergent level to be explanatorily autonomous. The core of both strong diachronic emergence and weak diachronic emergence is a focus on unpredictable emergent entities, which are new properties or new structures generated from evolution, and a characteristic emphasis on the diachronicity of the generation of emergent entities. Therefore, diachronic emergence has characteristics such as novelty in evolution, unpredictability, and autonomy of macro-explanation.