In this work, we propose an obstacle model to be used while simulating wireless sensor networks. To the best of our knowledge, this is the first time such an integrated and systematic obstacle model appears. We define several types of obstacles that can be found inside the deployment area of a wireless sensor network and provide a categorization of these obstacles, based on their nature (physical and communication obstacles), their shape, as well as their nature to change over time. In light of this obstacle model we conduct extensive simulations in order to study the effects of obstacles on the performance of representative data propagation protocols for wireless sensor networks. Our findings show that obstacle presence has a significant impact on protocol performance. Also, we demonstrate the effect of each obstacle type on different protocols, thus providing the network designer with advice on which protocol is best to use.