Main conclusion: The review provides information on the mechanisms underlying the development of spontaneous and pathogen-induced tumors in higher plants. The activation of meristem-specific regulators in plant tumors of various origins suggests the meristem-like nature of abnormal plant hyperplasia. Abstract: Plant tumor formation has more than a century of research history. The study of this phenomenon has led to a number of important discoveries, including the development of the Agrobacterium-mediated transformation technique and the discovery of horizontal gene transfer from bacteria to plants. There are two main groups of plant tumors: pathogen-induced tumors (e.g., tumors induced by bacteria, viruses, fungi, insects, etc.), and spontaneous ones, which are formed in the absence of any pathogen in plants with certain genotypes (e.g., interspecific hybrids, inbred lines, and mutants). The causes of the transition of plant cells to tumor growth are different from those in animals, and they include the disturbance of phytohormonal balance and the acquisition of meristematic characteristics by differentiated cells. The aim of this review is to discuss the mechanisms underlying the development of most known examples of plant tumors.