Reliable modeling of the flow induced coalescence is a very important step in prediction and control of the phase structure evolution during mixing and processing of immiscible polymer blends. This contribution is focused on recent results obtained in calculations of the probability, Pc, that the droplet collision is followed by their fusion. The calculations have been performed by the method of switching between the equations for drainage of the matrix trapped between spherical or highly flattened droplets. It has been found that Pc is a function of the viscosity ratio of the dispersed phase and matrix, p; it is independent of other system parameters for small droplet radii, R, and/or deformation rates. For a certain R or deformation rate, Pc starts decreasing steeply to a negligible value. The starting point and shape of this decrease depend on the choice of the equation describing the matrix drainage between flattened droplets. A new equation for the matrix drainage between approaching flattened droplets, applicable in the whole range of p, is proposed. The theory has been extended to systems with viscoelastic matrixes. Pc decreases with increasing relaxation time of the matrix. This decrease is small for short relaxation times but pronounced for relaxation times longer than 1 s. The comparison of assumptions of available theories of coalescence with characteristics of typical polymer blends shows that the effects of shape anisometry and elasticity of droplets and of inter-droplet interactions in blends with a high content of the dispersed phase should be studied to derive a reliable theory. Acknowledgement: The authors are grateful to the Grant Agency of the Czech Republic for financial support by Grant No. P106/11/1069.