We consider the computational complexity of reconfiguration problems, in which one is given two combinatorial configurations satisfying some constraints, and is asked to transform one into the other using elementary transformations, while satisfying the constraints at all times. Such problems appear naturally in many contexts, such as model checking, motion planning, enumeration and sampling, and recreational mathematics. We provide hardness results for problems in this family, in which the constraints and operations are particularly simple.More precisely, we prove the PSPACE-completeness of the following decision problems:• Given two satisfying assignments to a planar monotone instance of Not-All-Equal 3-SAT, can one assignment be transformed into the other by single variable "flips" (assignment changes), preserving satisfiability at every step?• Given two subsets of a set S of integers with the same sum, can one subset be transformed into the other by adding or removing at most three elements of S at a time, such that the intermediate subsets also have the same sum?• Given two points in {0, 1} n contained in a polytope P specified by a constant number of linear inequalities, is there a path in the n-hypercube connecting the two points and contained in P ?These problems can be interpreted as reconfiguration analogues of standard problems in NP. Interestingly, the instances of the NP problems that appear as input to the reconfiguration problems in our reductions can be shown to lie in P. In particular, the elements of S and the coefficients of the inequalities defining P can be restricted to have logarithmic bit-length.extension of Schaefer's dichotomy to the connectivity of Boolean satisfiability due to Gopalan et al. [9], and an overview of the complexity of reconfiguration problems by Ito et al. [16]. In the canonical example of Boolean satisfiability, one is given two satisfying assignments to connect by a sequence of variable assignment flips, such that the formula remains satisfied at every step. The study of this type of question also benefits from the interest of puzzle designers and recreational mathematicians; token-sliding problems, for instance, are related to the famous 15-puzzle, popular in the late 19th century [30]. Combinatorial reconfiguration now constitutes a quickly developing field with dedicated research groups and workshops (such as the combinatorial reconfiguration workshop held in Banff in January 2017). For a more thorough survey and history of this family of problems, we refer to van den Heuvel [13].Reconfiguration of independent sets in graphs is among the most studied problem in this vein (see [4,6,8,14] for recent results) and is relevant to our findings. In these problems, one is given a graph G and two independent sets of G of the same size k, and the goal is to transform one into the other using elementary operations, preserving independence at every step. The operations consist either of "token slides", in which a vertex in the independent set is replaced by one of its neighbors...