Semiconductor nanocrystals are molecular-like semiconductor materials composed of hundreds to thousands of atoms, intermediate between a solid and a molecule. In this regime fundamental properties depend on size, composition and shape. Due to the tunable properties of such nanostructures, they show great potential for use as building blocks of devices in nanotechnology. In our lab we are interested in preparation, characterization, and in the optical and electronic properties of nanocrystals, while also developing new applications based on their unique properties. Special attention is devoted to II-VI and III-V semiconductors nanostructures of technological relevance, examining the properties of a single nanostructure as well as their collective effects in assemblies. In recent years we focus on development of hybrid nanoparticles, composed of disparate elements such as metal-semiconductor hybrid nanoparticles. Such systems manifest a unique model for the semiconductor-metal interface on the nanoscale. Recently, we also developed synthetic approaches for the controlled preparation of coupled nanocrystals structures, which demonstrate coupling effects already at room temperature. The spectroscopic research in this area is tightly accompanied by model calculations of the expected coupling effects and their trends. The high level of control of nanoparticle synthesis and deep understanding of the physico-chemical properties achieved in recent years opened avenues for application of such nanoparticles in solar energy harvesting, printed electronics, display, bio-imaging and bio-medical applications, 3D printing, and more.