In this paper a review of recent developments on safety and runaway prevention in batch and semibatch reactors is given. To prevent thermal runaways in chemical reactors usually three lines of defense have to be considered: choice of the right operating conditions, early warning detection system and system to handle running away reactions. Because of its too specific nature, the last line of defense is not discussed in this review, while the former two are considered in detail. Firstly the capability and applicability of the reaction calorimeter for safety assessments are reviewed, then fundamental safety criteria for safe operation of batch and semibatch reactors are reported and discussed. A review of the most useful and efficient methods for a correct choice of safe operating conditions is split into two cases: one, when the reaction kinetics are known, and when kinetic information is not available. Also several promising recent developments of an early warning detection system for the on-line detection of unexpected situations leading to a thermal runaway are presented.