Necrosis plays a fundamental role in plant physiology and pathology. When plants or plant cell cultures are subjected to abiotic stress they initiate rapid cell death with necrotic morphology. Likewise, when plants are attacked by pathogens, they develop necrotic lesions, the reaction known as hypersensitive response. Great advances in the understanding of signaling pathways that lead to necrosis during plant-pathogen interaction have been made in the last two decades using Arabidopsis thaliana as a model plant. Further understanding of these signaling pathways, as well as those regulating the execution phase of necrotic cell death per se would require a robust set of readout assays to detect and measure necrosis in various plant model systems. Here we provide description of such assays, beginning from electron microscopy, as the "gold standard" to diagnose necrosis. This is followed by two groups of biochemical and cytochemical assays used by our group to detect and quantify mitochondrial dysfunction and the loss of protoplast integrity during necrosis in Arabidopsis plants and cell suspension cultures of both Arabidopsis and Norway spruce.