Molecular assembly plays a critical role in constructing the physical world beyond molecules. Living organism shows the highest level of molecular assembly, inside which complex subunits assemble into functional assemblies through complex intermolecular interactions. Self-assembly is the dominant paradigm in the research of artificial molecular assembly, whereas most molecular assemblies in living organism are multi-pathway, multi-step, and hierarchical, thus requiring the assistance and guidance of other molecules. Among these assisting methods, catassembly uses catassemblers to modulate the kinetics and pathways of the assembly processes, exhibiting high efficiency and selectivity. In living organisms, catassemblers such as molecular chaperones, histone chaperones and allosteric factors are critical to protein folding, chromatin remodeling, and regulation of enzyme activities, respectively. In this review, we analyzed and then summarized various catassembly cases in living organisms, and envisioned that catassembly will be more important in more complex molecular assemblies. These investigations may provide a new perspective for the development of catassembly in artificial and living systems, and further improve the complexity, controllability and function of molecular assembly to a higher level.