The potential of physical methods for in situ studies of the phase stability and physicochemical evolution of heavy oils is analyzed. The basic principles of in situ application of the methods are outlined; the scope and limitations of each approach are discussed. Particular attention is paid to the visualization of the asphaltene aggregation process using attenuated total reflection Fourier transform IR spectroscopy and magnetic resonance imaging, as well as small-angle X-ray and neutron scattering, electron spin resonance and electron and optical microscopy. These methods provide complementary information on the properties and behaviour of oil disperse systems on various spatial and temporal scales ranging from the rotational mobility of asphaltene molecules and the dynamics of their local environment with a characteristic time of ~10-10 s and evolution of the size characteristics and shape of asphaltene aggregates to visualization of asphaltene aggregation and formation of precipitates in crude oils and their blends with spatial resolution from a few to thousands of micrometres with a characteristic time from seconds to hundreds of hours. The approaches described can be efficiently applied in a wide range of temperatures and pressures, as well as in the presence of chemicals that affect the stability of heavy oils. The bibliography includes 241 references.