Polymer crystallization often occurs in the presence of foreign bodies, such as walls of processing tools. In such cases, the main problem is the competition between nucleation in the bulk polymer and nucleation on well-identified surfaces. If many nuclei are activated at the surfaces, their proximity imposes that entities emanating from these nuclei grow preferentially normal to the surface, leading to transcrystalline zones. The competition between surface and bulk nucleation can be studied through crystallizations of thin polymer films in contact with pan surfaces in a DSC apparatus. These experiments show that important transcrystalline regions correspond to coarse spherulites at the specimen core. Conversely, thin transcrystalline regions are associated to a large number of small spherulites at the core. The total thickness of the transcrystalline zones and the maximum diameter of the bulk spherulites both increase up to a plateau value of the same order of magnitude. Thus, in thin samples transcrystallinity is limited by the sample thickness. When thickness increases, the transcrystalline zones can grow, but up to a limiting value, because at a certain stage their development is stopped by the growth of bulk spherulites. A specific analysis of these DSC experiments has been proposed. Using thin completely transcrystalline specimens, simple derivations give rise to the number of nuclei per unit surface that promote transcrystallinity and to the growth rate of the polymer as a function of temperature. Measurements performed with different thicknesses allow us to determine the crystallisation kinetics of the polymer itself, that is, not disturbed by transcrystallinity. This analysis of transcrystallinity has been applied to different polymers, including polyamide 6-6.