This paper introduces in a natural way a notion of horizontal convex envelopes of continuous functions in the Heisenberg group. We provide a convexification process to find the envelope in a constructive manner. We also apply the convexification process to show h-convexity of viscosity solutions to a class of fully nonlinear elliptic equations in the Heisenberg group satisfying a certain symmetry condition. Our examples show that in general one cannot expect h-convexity of solutions without the symmetry condition.1.1. Background and motivation. In order to explain the motivation of our work, let us first briefly recall the definition, properties and several applications of the convex envelope in R N . For any given function u ∈ C(R N ) that is bounded below. There are at least two ways to define the Euclidean convex envelope, which we denote by Γ E u. The first is to consider the largest convex function majorized by u, that is,An equivalent way of defining the convex envelope is to convexify pointwise the given function u; namely, we havefor all p ∈ R N . Compared to (1.1), the definition in (1.2) is more constructive and more likely to be used in practical computations.Besides the equivalent definitions above, there is a characterization of the convex envelope in terms of a nonlinear obstacle problem, recently proposed by [35,36]. More