Over the last decade there has been a significant of interest of researchers in ultra-cold matter physics in polar molecules having a permanent electric/magnetic dipole moment. The long-range nature of dipole interactions and the possibility to control them using external electric or magnetic fields can be applied to control chemical reactions in ultra-cold gas, to process quantum information, to study fundamental properties of matter and few or many bodies physics problems. In order to plan and carry out the above mentioned research it is necessary to obtain the trapped ultracold (~uK/nK) or quantitatively degenerated gas of polar particles in the ground rovibrational state. Obtaining a molecular gas of such parameters under laboratory conditions is possible by associating the molecules from the trapped ultracold atomic gas or by using direct laser cooling of the moleculesThese methods of obtaining ultra-cold molecules in the desired energy state require a very precise knowledge of energy structure of molecules. This includes both information on the energy of certain rovibrational levels and the probability of transitions between the levels involved in cooling/transfer scheme, i.e. Franck-Condon factors, transition dipole moments. The results of spectroscopic studies which provide such information about the electronic structure of selected polar molecules will be presented during the lecture.