This thesis, which consists of five articles in English and a summary in Swedish, deals with observations and theoretical studies of dark interstellar clouds, and with a method for calculations of radiative transfer. Article 1 is a catalogue of bright nebulosities in dark clouds, based on a survey of photographic sky atlases. Such nebulosities can be regarded as possible indicators of star formation in the clouds. Article 2 is a theoretical investigation of the transfer of ultraviolet radiation and the formaldehyde abundance in the p Ophiuchi cloud complex. An expression for a source function that includes anisotropic scattering is derived. Article 3 describes a procedure for the solution of non-LTE, multi-level, radiative transfer problems using the Monte Carlo method. A technique for reducing the random fluctuations inherent in the Monte Carlo method is also introduced. The procedure is applied to a study of carbon monoxide line radiation in a model of a spherical dark cloud. A listing of the FORTRAN program that was written for this application is presented in article 4. The program contains numerous comments, intended to make it selfexplanatory. Article 5 summarizes observations of the 6-cm, 2-cm and 2-mm formaldehyde lines in the L1551, L1317, NGC7023 and L134 cloud regions. The Monte Carlo procedure has been applied for model studies of L1551. These studies, as well as an analysis of the hyperfine structure of the 6-cm line, indicate that both temperature and density increase towards the centre of the cloud.