J-aggregation of cyanine dyes is a bright example of self-organization of matter. Traditional methods of J-aggregation have one essential defect. As a result of random growth of aggregates the assembly of units of a various size is formed which reduces their photochemical activity. The method of controlled formation of J-aggregates of cyanine dyes of betaine structure from dimers is considered. The method is called as "block building " of J-aggregates. The study of an equilibrium between the various forms of anionic cyanine dyes in water solutions has allowed to make paradoxical, on the first sight, conclusion, that a method of "block building " of J-aggregates from dimers are the most preferable for construction of "ideal" J-aggregates of the same size (monostructural J-aggregates). It was shown for thiamonomethine and trimethinecyanines, that the J-aggregates consist from two dimers, i.e. from 4 molecules of dye. Use of the method of the "block building" of J-aggregates has allowed to receive the mixed J-aggregates from cyanine dyes with a various length of the polymethine chain. The fundamental conclusion is made that by a modification of concentration of the dyes included in a mixed J-aggregate it is possible to vary a maximum of absorption in a broad interval. Is shown, that the J-aggregates obtained a method of the «block building» have very narrow absorption bands of light and also high intensity of photoluminescence and electroluminescence. The developed method of "block building" is an effective way of the formation of the monosized J-aggregates as well as the J-aggregates from dyes essentially differed structurally. It opens the way for formation of complicate organised structures with new electronic and optical properties for molecular electronics and optoelectronics.