In this review, we highlight particle based crystallization pathways leading to single crystals via mesoscopic transformation. In contrast to the classical mechanism of atom/molecule mediated growth of a single crystal, the particle mediated growth and assembly mechanisms are summarized as "non-classical crystallization", including exiting processes like oriented attachment and mesocrystal formation. Detailed investigations of non-classical crystallization mechanisms are a recent development, but evidence for these pathways is rapidly increasing in the literature. A major driving force for these investigations originates from biomineralization, because it seems that these crystallization routes are frequently applied by natural organisms. We give a non-exhaustive literature survey on these two mechanisms with a focus on recent examples and studies, which are dedicated to a mechanistic understanding. Furthermore, conditions are introduced for which these non-classical crystallization mechanisms can be expected, as they are always an alternative reaction pathway to classical crystallization.