This chapter outlines nature of stimulus/injury and cellular responses that includes cellular adaptations, cell injury, intracellular accumulations and pathologic calcification. Cells can adapt to certain pathologic stimuli by changes in size, number or differentiation of cells in affected tissue. These are reversible changes and constitute cellular adaptations. Various adaptations such as hyperplasia, hypertrophy, atrophy and metaplasia have also been discussed in detail. Metabolic derangements may result in retention or excess accumulation of various substances inside the cells. Various diseases occurring due to these arrangements have been provided in detail.