R package unfoldr: Stereological Unfolding for Spheroidal Particles. Stereological unfolding as implemented in this package consists in the estimation of the joint size-shape-orientation distribution of spheroidal shaped particles based on the same measured quantities of corresponding planar section profiles. A single trivariate discretized version of the (stereological) integral equation in the case of prolate and oblate spheroids is solved numerically by the EM algorithm. The estimation of diameter distribution of spheres from planar sections (Wicksell’s corpuscle problem) is also implemented. Further, the package provides routines for the simulation of a Poisson germ- grain process with either spheroids, spherocylinders or spheres as grains together with functions for planar sections. For the purpose of exact simulation a bivariate size-shape distribution is implemented.