A complete set of stages occurring during flow-induced crystallization of polymers is discussed. The first stage is the stretching of the molecules described by the condition that the shear rate has be above inverse Rouse time of the longest polymer molecules in the polymer ensemble. The second stage is the collisions of the stretched molecules resulting in the formation of stable oriented point nuclei described by the critical specific work. And, finally, the third stage is the transformation of the rows of the oriented point nuclei into a fibrillar (shish) structure described by the critical strain. The effect of both processing conditions and polymer molecular weight distribution on the flow-induced nucleation (FIN) in polyolefins is analysed by rheo-optical techniques and small angle X-ray scattering methods. Results obtained for polymers of industrial grade as well as bi-modal and tri-modal polyethylene blends composed of low polydisperse low-, medium- and high- molecular weight polymer chains are presented. It is demonstrated that a shear rate dependence of the critical specific work parameter for the onset of FIN in polydisperse polymers is controlled by the molecular weight distribution and that the longest chains (high molecular weight fraction of the ensemble) of the polymer mainly dominate the process. The role of the matrix polymer (low molecular weight fraction of the ensemble) has also been investigated. It was found that the critical specific work measured for the onset of FIN in the blends has a power law dependence on the molecular weight of the matrix polymer. This indicates that the matrix polymer can affect the stretching of the long chains and limit their ability to act as nucleation sites for flow-induced crystallization.