Firesetting has significant social and financial consequences. Firesetters are deemed a dangerous group with a poor prognosis. The existing literature has focused on describing the characteristics of firesetters and developing typologies. The psychological understanding of firesetting is limited. Implicit theories are underlying causal theories. Positioned within a social cognitive framework they are thought to be the intervening process between observable stimuli and responses which enable individuals to make sense of their social world and make predictions within it. The identification of implicit theories has contributed to the understanding of the way information processing is carried out by other offender groups. This study aimed to explore the implicit theories held by male firesetters in secure forensic psychiatric services. Using grounded theory methodology, interviews were conducted with nine men with a history of firesetting. Six implicit theories were generated: malevolent world, uncontrollable world, violence is normal, accountability, fire is controllable and fire is a powerful tool. These implicit theories have the potential to be utilised as treatment targets by challenging and restructuring them. Future research should focus on exploring the specific implicit theories at different points in the offense process, those held by subtypes of firesetters, and those held by female firesetters.