An important step in the development of drugs is the selection of an appropriate solid form of active pharmaceutical ingredient (API). Each solid form exhibit different physicochemical properties and this is determined by structural parameters. Pharmaceutical cocrystals, as well as molecular salts, have gained great interest among crystal engineers, chemical and pharmaceutical researchers, because they significantly diversify crystal forms that exist for a particular API. Both, cocrystals and molecular salts, in many cases improve physicochemical properties and indirectly affect the bioavailability of API. Literature review (PART I) is addressed to the crystal engineering of pharmaceutical cocrystals and salts from a perspective of design, prediction, preparation and improvement of physicochemical properties (solubility, stability and bioavailability). The PART II is devoted to novel pharmaceutical cocrystals of pentoxifylline with altered physicochemical properties with respect to API. To obtain new crystal forms and to get insight into the driving forces, guiding cocrystallization, an experimental and in silico screening of pentoxifylline with pharmaceutically acceptable coformers have been performed. The free energy of experimental and hypothetical crystal structures has been calculated in order to estimate the relative stability. Six cocrystals of pentoxifylline have been obtained and characterized by physical methods. The experimental and in silico results were found to match very well. Strong correlation between melting points of pentoxifylline cocrystals and corresponding coformers has been detected. A significant decrease in solubility of pentoxifylline cocrystals as compared to pure pentoxifylline was observed. The PART III describes preparation and properties of diltiazem, sildenafil and propranolol molecular salts with altered physicochemical properties in respect to API. Three diltiazem, one sildenafil and eight propranolol molecular salts were synthesized and characterized. In this part the crystal engineering approach has been employed to obtain crystal forms with desired solubility, thus the design and preliminary characterization of pharmaceutical molecular salts are discussed. Keywords: Supramolecular chemistry, crystal chemistry and engineering, pharmaceutical cocrystals, pharmaceutical molecular salts, pentoxifylline, diltiazem, sildenafil, propranolol, single crystal X-ray crystallography, powder X-ray diffraction, physicochemical properties.