Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation device that offers promise of more effective and longer lasting rehabilitation effects and a tool that can be used to causally manipulate brain function. One area that has reaped particular benefit is in the study of language processing in adults with and without brain damage. In the face of inconsistent findings, and the dawning realisation that tDCS is not as easy to use as first thought, more attention to the basic questions of how and when tDCS can affect language processing is needed. The general aim of this thesis was to better understand how changing states within the language network affected the response to tDCS in healthy adults. This thesis extends previous reported effects of tDCS on confrontation naming and word learning, by examining the impact of participant, task, and stimulus-level characteristics on expected tDCS effects in healthy adults. The main findings were: 1) tDCS did not facilitate response times during confrontation naming, irrespective of the activation state of specific stimuli, or region of interest targeted 2) bilingual experience facilitates word learning but phonological memory abilities and language switching abilities cannot explain this effect, highlighting the potential impact of experiential and cognitive factors on task performance and 3) the effects of stimulation on word learning are likely to be much weaker than previously reported, at least in single sessions, and may interact with participant, task and stimulus-level characteristics in a complex manner. It is too early to abandon tDCS as a neuroscience tool but careful consideration of experiential, cognitive, and biological factors that may interact with task performance need to feature more heavily in the design and execution of future studies. Code and data for each of the chapters where this is relevant can be accessed via the Open Science Framework site for this thesis (OSF site: http://bit.ly/JPayne_PhD_OSF).