The formation of dispersoids of nanometer size particles and its consequences on the following properties is discussed: precipitation hardening, stabilisation of grain size, stabilisation of austenite against martensitic, and induction of R-phase transformation. Firstly, a survey is given on the aspects which have to be considered for the production of evenly distributed particles in the size range 1–10 nm by precipitation: pre-nucleation clustering, size of the elementary cell, interfacial structure and defects in the matrix lattice, and in situ transformation of metastable phases. Rules are proposed for the production of nano-dispersoids which are valid for all types of alloys. Interaction of dislocations with the dispersed particles leads to a quantitative understanding of the amount and upper limit of precipitation hardening and related properties: work-hardening, localisation of strain, and crack-growth. “Hardening” of ferromagnetic and superconducting materials can be analysed in a similar manner. Interaction of moving grain boundaries with the particles provides the base for highly retarded grain growth. This is required for many heat treatments, for example, to control grain size and texture. Finally, dispersed particles can impede the motion of the interface of an austenite → martensite transformation. In this way, the transformation temperature of austenite can be lowered considerably in steels as well as in shape memory alloys. On the other hand, stress fields or concentration gradients in the environment of particles may favour the formation of pre-martensitic effects like R-phase transformation. It is attempted to provide a unifying and quantitative approach on the formation of ultrafine dispersoids and their effect on a multitude of properties.