The thesis to be proved is that the mechanism of optical interstellar polarization, preferential extinction by magnetically aligned aspherical grains, can be extrapolated successfully to explain the large infrared polarizations observed in molecular cloud protostellar sources. Observations of the linear polarization of 13 sources between 1.65 μm and 4.8 μm and of the circular polarization of 9 sources at 2.2 μm have been obtained. Analogously to the optical interstellar case, the data have been modelled using the Rayleigh approximation to calculate extinction and phase -lag efficiencies for a number of grain models. The models successfully account for the high infrared ellipticities (ratio of linear to circular polarization) observed in the molecular clouds, with twists in the grain alignment of about 40°; smaller than those required to explain optical interstellar circular polarization. A shortcoming in the model linear polarization at X < 3 μm is attributable to a failure in the Rayleigh approximation. A dissimilarity in the polarization through the ice band between two of the protostellar sources can be understood by differing grain compositions. A correlation between the position angles of polarization of the protostars and the nearby interstellar field stars can be interpreted by saying the Galactic magnetic field permeates the dense molecular clouds. There are indications that the polarization mechanism even operates in the enhanced density regions of the clouds; then a more efficient alignment mechanism than paramagnetic relaxation is required. It is suggested that this may be "pinwheeling," possibly accompanied by super-paramagnetism of the grains. The twists in the magnetic field lines implied by the model may arise in the collapse process of the rotating, magnetized clouds.