Induced by proteins within the cell membrane or by differential growth, heating, or swelling, spontaneous curvatures can drastically affect the morphology of thin bodies and induce mechanical instabilities. Yet, the interaction of spontaneous curvature and geometric frustration in curved shells remains poorly understood. Via a combination of precision experiments on elastomeric spherical shells, simulations, and theory, we show how a spontaneous curvature induces a rotational symmetry-breaking buckling as well as a snapping instability reminiscent of the Venus fly trap closure mechanism. The instabilities, and their dependence on geometry, are rationalized by reducing the spontaneous curvature to an effective mechanical load. This formulation reveals a combined pressurelike term in the bulk and a torquelike term in the boundary, allowing scaling predictions for the instabilities that are in excellent agreement with experiments and simulations. Moreover, the effective pressure analogy suggests a curvature-induced subcritical buckling in closed shells. We determine the critical buckling curvature via a linear stability analysis that accounts for the combination of residual membrane and bending stresses. The prominent role of geometry in our findings suggests the applicability of the results over a wide range of scales.PACS numbers: 02.40. Yy, 87.17.Pq, 87.10.Pq Owing to their slender geometry, thin elastic shells display intriguing mechanical instabilities. Perhaps the most iconic example is the buckling of a spherical shell under pressure -a catastrophic situation that often leads to structural failure [1,2]. Instabilities and shape changes are also fundamental during the development and morphogenesis of thin tissue [3,4]. To control and evolve shape, Nature heavily relies on internal stimuli such as growth, swelling, or active stresses [5,6]. If the stimulus varies through the thickness of the shell, it generally induces a change of the spontaneous (or natural) curvature of the tissue [7]. Examples are the ventral furrow formation in Drosophila [8] or the fast closure mechanism invoked by the Venus fly trap to catch prey [9]. Harnessing similar concepts for technological applications, internal stimuli were also suggested as a means to design adaptive metamaterials [10] and soft robotics actuators [11]. To describe the mechanics of slender structures with arbitrary stimuli, classical shell mechanics was extended recently to model bodies that do not possess a stressfree configuration [12][13][14], leading to the non-Euclidean shell theory [15]. Despite recent progress [16,17], the role of curvature-altering stimuli, and their interplay with geometric frustration and instabilities in thin, initially curved shells, remains poorly understood.In this Letter, we combine precision experiments with non-Euclidean shell theory to reveal how curvature stimuli induce mechanical instabilities in spherical shells. Our experiments demonstrate symmetry-breaking as well as snap-through shape transitions depending on the amoun...