The usage of molecular clusters as a basis of molecular electronic systems is considered experimentaly and theoreticaly. The technique of formation of the molecular structures based on the organometallic nanocluster molecules using Langmuir-Blodgett (LB) technology is described. This technique allows to create reproducibly the nanostructures with preset structural characteristics: separate clusters, one-dimensional chains of clusters, two-dimensional regular arrays of clusters.. The structural study as well as electron transfer characteristic measurements was made by scanning tunneling microscope (STM) for a large number of cluster molecules that are chemically different and variable in composition. Asymmetric clusters are shown to be better for this technology. The single-electron (correlated) tunneling (SET) effect is studied in the systems based on the single organometallic nanocluster molecules. A molecular single-electron transistor on the base of a single cluster molecule operating at room temperature was realized. An analysis of I-V curves and control curves of such transistors with various molecules as a central electrode have shown that the atomic and electronic structure of nanoclusters containing in the core from 3 to 23 metal atoms have no crucial importance for the realization of the transistor effect in itself. The charge sensitivity of the systems at room temperature was estimated from measured parameters of transistors. It appears to be about 10-4 e/Hz1/2 at room temperature which is close to the typical values for metallic thin-film low-temperature (100 mK) single-electron systems. The planar molecular nanosystems with arrays of molecules in the narrow gaps (less than 10 nm) between the metal electrodes were implemented. Study of electron transport through such systems at room temperature have shown the correlated character of electron tunneling in such systems It show the possibility of design of practical molecular SET devices. I-V curves of molecular SET transistor are simulated based on a modified theory of single-electronics that accounts for the discreteness of the energy spectrum of the molecule. This simulation was performed including for the first time the effects of energy relaxation of the electrons in the molecule for two limiting cases of fast and slow relaxation, and for two types of the molecule energy spectrum. A comparison of the simulated I-V curves with the experimental ones allow to conclude that the experimental conditions correspond to the slow energy relaxation case.This work was supported in part by INTAS-99-00864, ISTC (Gr. No.1991) RFBR (99-03-32218, 01-02-16580).