Noble-metal nanocrystals have attracted increasing attention owing to their potential use in catalysis, electronics, and biology.[1] The physicochemical properties of these nanocrystals are highly sensitive to their shape and size. For example, the number, location, and intensity of surface plasmon resonance (SPR) bands of Au and Ag nanocrystals display a strong correlation with the shape of the particle.[2] Furthermore, the reactivity and selectivity of metal nanocatalysts depend strongly on the crystallographic planes exposed on the surface of the particles and can therefore be tuned by controlling the morphology of these particles.[3] An exquisite shape control of noble metal nanocrystals is therefore highly desirable for tailoring their properties and is also required for high performance in many applications. Palladium nanocrystals are widely used as primary catalysts for the low-temperature reduction of automobile pollutants, hydrogenation reactions, and organic reactions such as Suzuki, Heck, and Stille coupling.[4] Pd is also wellknown for its remarkable capacity in hydrogen absorption.[5] Most of these applications are related to the adsorption of hydrogen onto the surface of Pd nanocrystals. Recent studies have revealed that the hydrogen-absorption capacity and surface-enhanced Raman scattering (SERS) activity of Pd nanocrystals are dependent on their shape.[6, 7] A wide variety of Pd nanocrystals, including cuboctahedra, cubes, rods, and bars, have been prepared to date, mostly by the polyol method, in which ethylene glycol (EG) serves as both a reductant and a solvent.[8] However, the major products of a polyol synthesis are often restricted to shapes such as truncated cubes or cuboctahedra, owing to the fast reduction and growth rate associated with the polyol process. An alternative water-based system could provide a more convenient and environmentally benign route to the synthesis of noble-metal nanocrystals, because it does not involve toxic organic solvents or reagents. Recently, our group reported the syntheses of Pd thin plates and icosahedra in aqueous solution using poly (vinyl pyrrolidone)(PVP) and citric acid as the reducing agent, respectively.[9, 10] It is worth pointing out that an octahedron and a decahedron are two particle shapes that face-centered cubic (fcc) noble metals can potentially take, although high-yield syntheses of these two types of nanostructures are yet to be realized for Pd.[11] The formation of a particular shape in the synthesis of metal nanocrystals is often explained in terms of the presence of surfactants or capping agents,[8, 10] which can change the order of free energies of different facets through their interactions with the metal surface in a solution-phase synthesis.[12] This alteration may significantly affect the relative growth rates of different facets and thus lead to different morphologies for the final products. To achieve shape control of a nanocrystal, however, not only the thermodynamics or physical restrictions imposed by the surface stabilizing agent must be considered, but also nucleation and kinetics. Herein, we report on a water-based system for the facile synthesis of Pd nanocrystals with various shapes by reducing a Pd precursor with citric acid. Citric acid or citrate ion can also serve as a capping agent in this system thanks to their strong binding to the {111} facets of Pd.[10, 13] More specifically, we demonstrate that citric acid favors the formation of a structure such as an octahedron, icosahedron, or decahedron whose surface is covered by {111} planes. We also demonstrate that the shape of the Pd nanocrystals can be controlled by varying the concentrations of the Pd precursor and citric acid. We …