Data transport is a core function for Wireless Sensor Networks (WSNs) with different applications having varied requirements on the reliability and timeliness of data delivery. While node redundancy, inherent in WSNs, increases the fault tolerance, no guarantees on reliability levels can be assured. Furthermore, the frequent failures within WSNs impact the observed reliability over time and make it more challenging to achieve the desired reliability. Unfortunately, a framework for modeling reliability of data transport protocols in WSNs is currently missing. The existence of such a framework would simplify evaluation, comparison and also adaptation of these protocols. We formulate the problem of data transport in a WSN as a set of operations carried out on raw data. The operations aim at filtering the raw data to streamline its reliable transport towards the sink. Based on this formulation we systematically define a reliability framework. This paper argues for the usefulness of the reliability framework by classifying existing transport protocols and comparing their reliability. © 2007 IEEE.