A change of intracellular calcium concentration is an early event in a large array of biological processes in plants, such as cell division, polarity, growth and development at normal conditions and under adaptation to abiotic and biotic stresses. This chapter will focus on calcium signaling induced by different types of abiotic stress, such as salt, cold, anoxia, aluminium and heavy metal stresses. A minor part deals with biotic stress signaling. Most investigations, so far, concerned Ca²⁺ signaling in the cytosol, but the last years also signaling in the nucleus and other cell compartments such as mitochondria, ER and cell wall have been reported. We will compare the specific “signature” of calcium, including duration, amplitude and frequency of the signaling, which is induced by different stresses and is important for a change of the physiological function. Different stores for calcium take part in the signaling under various types of stresses. Of special interest is a comparison of signaling in tolerant and sensitive species and cultivars.