In this work, we introduce the class of Interrupt Timed Automata (ITA), which are well suited to the description of multi-task systems with interruptions in a single processor environment. This model is a subclass of hybrid automata. While reachability is undecidable for hybrid automata we show that in ITA the reachability problem is in 2EXPSPACE and in PSPACE when the number of clocks is fixed, with a procedure based on a generalized class graph. Furthermore we consider a subclass ITA- which still describes usual interrupt systems and for which the reachability problem is in NEXPTIME and in NP when the number of clocks is fixed (without any class graph). There exist languages accepted by an ITA- but neither by timed automata nor by controlled real-time automata (CRTA), another extension of timed automata. However we conjecture that CRTA is not contained in ITA. So, we combine ITA with CRTA in a model which encompasses both classes and show that the reachability problem is still decidable.