In this work the development of a low-temperature scanning probe microscope and the investigation of thin insulating films at the atomic limit is presented. The scanning probe microscope has been designed in a modular way to provide large flexibility for the implementation of different types of measurements. The design provides high stability as well as a large field of view. In the present stage of development the instrument is used as a scanning tunneling microscope which has been used for all experiments presented. Two different systems of insulating films at the atomic limit have been investigated for the first time: KCl(100) on Cu(110) and KCl(100) on Cu(111). Depending on the deposition parameters KCl(100) on Cu(110) shows different types of growth. This includes step decoration and modification at low coverage as well as growth of islands with monolayer or bilayer height at higher coverage. The islands appear with a stripe structure in STM. In the case of bilayer islands atomic resolution was achieved and the stripe structure has been identified as an electronic Moirée-pattern caused by the incommensurable lattice constants of salt and copper. The growth is not perfect, hence it was possible to identify defects like grain boundaries and point defects. KCl(100) on Cu(111) has been shown to grow in large islands or closed layers. Similar to the bilayers on Cu(110) atomic resolution has been achieved. This system provides an interface state which is related to the Cu(111) surface state. The interface state shows nearly the same dispersion relation as the surface state of pristine copper but with a different onset energy. This dispersion relation is discussed in the framework of different dielectric layers on metal substrates. Different electronic structures have been observed for the salt islands on both substrates: field resonances and an intrinsic salt state. Both systems have been shown to be transparent for electron tunneling within their bandgap. The latter has been determined for salt bilayers on Cu(110). Compared to the case of pristine copper, the image states on the pristine copper surfaces are shifted to lower energies. This energy shift is discussed in detail. The growth of cobalt phthalocyanine on Cu(110) and Cu(111) at low coverage has been studied. In the case of Cu(111) the center of the molecule has been identified as the main scattering center.