This chapter focuses on nonlinearities in solid-state spectroscopy (fluorescence) detectors. These are the most common counting detectors found in X-ray absorption spectroscopy (XAS) applications. The emphasis is on nonlinearities because they define the limits of XAS investigations. For example, chemical speciation using linear combination fitting of X-ray absorption near-edge structure (XANES) relies on good linearity across large signal ranges. Nonlinear damping of features around an absorption edge (especially strong white lines) can lead to the misinterpretation of data. In the extended X-ray absoption fine-structure (EXAFS) regime, amplitude variations are small and therefore nonlinearities of small magnitude may be less problematic. However, in some cases monochromator glitches may not normalize out and leave spurious signals at the level of EXAFS amplitudes. The key nonlinearity effects discussed in this chapter are detector dead time, pulse pile-up and fluorescence escape peaks, as well as charge sharing for multi-element detectors.