The effect of fluctuations of either thermodynamic or environmental origin on ignition in explosive systems is analyzed, with special emphasis on thermal explosion. A simple model due to Semenov is first analyzed in the zero-dimensional approximation. It is shown that the ignition times exhibit a wide dispersion, which at the level of the probability distribution of temperature shows up as a transient bimodality. Next, an extension to a spatially distributed system is developed. It is shown that fluctuations induce unexpected symmetry-breaking phenomena, reflected by a considerable dispersion of the position of the first "hot spot" initiated in the system.