General regularities of nanoparticles formation from solutions under various conditions (organic and aqueous media, mild and rigid conditions) are presented and analyzed. Two ways of forming a solid phase, based on simultaneous or sequential crystallization, are proposed; examples of the corresponding synthetic procedures are given. The process of sequential crystallization is described. The ability to reduce and increase particle sizes by using morphological regulating agents has been demonstrated by specific examples. Nucleating agents or nuclei can be successfully used to reduce the size of particles or to obtain the desired crystalline phases (if the substance is characterized by several modifications).