Coalescence (usually called sintering) of polymer particles is inherent in the process of rotational molding. Based originally on the coalescence of glass and ceramic particles, models were developed for the neck formation and growth. The driving force is surface tension opposed by viscosity. A model first proposed by Frenkel in 1945 (corrected by Eshelby in 1949) applies to the early stages of neck formation. This model was subsequently extended to the entire process of neck formation by the author and his co-workers (Bellehumeur, Pokluda , Kontopoulou and Takacs ) in the 1990s and early 2000s . Further studies also included the role of viscoelasticity. The experimentally observed trends are fully in agreement with the model predictions. Experiments in rotational molding showed the great usefulness of the modified Frenkel/Eshelby model. Experimental studies of coalescence of just two polymer particles under the microscope in a heated chamber can provide a lot of information on the role of various parameters such as particle size, particle shape and polymer rheology. These studies have found application in the determination of rotomoldability of various resins, including recent efforts on rotational foam molding (by Emami and the author in publications that appeared in 2014 and 2015). Recently, the extended Frenkel/Eshelby model has found applications in laser sintering (LS) in which an object is built layer-by-layer using powdered materials and a scanning laser beam. The next improvement of the model requires inclusion of temperature dependence of viscosity due to high variation in temperatures in LS. Among the benefits of accurate modeling in this area of additive manufacturing is the potential for enlarging the number of suitable materials. Of course,the properties and how they compare to parts produced by other methods is the ultimate arbiter of usefulness.