In this paper we explore methods for the online mapping of received radio signal strength with mobile robots and localizing the source of the radio signal. By utilizing Gaussian processes, we are able to build an online model of the signal-strength map that can, in turn, be used to provide the current maximum likelihood estimate of the source location. Furthermore, using the estimate of the source location, the Gaussian process model allows for prediction of received signal strength with confidence bounds in regions of the environment that have not been explored. Finally, we develop a control law for collecting samples of the signal strength with mobile robots that allows for online estimation of the radio signal source.