This paper aims to provide an overview of factors affecting the validity of epidemiological studies on health effects of mobile phone use. A qualitative review of published studies is presented, covering both risk assessment and exposure assessment. Considerable random error is likely to have occurred in studies carried out so far, primarily related to exposure assessment. Self-reported use of mobile phone appears to be imprecise. The relationship between the amount of mobile phone use and the radio-frequency field is unclear. Several factors affect the strength of the radio-frequency field emitted by the phone, and accommodating their effect has the potential to improve exposure assessment. The major opportunity to improve the quality of evidence is, however, through prospective studies. The major limitation of epidemiological studies addressing the health effects of mobile phone use is related to exposure assessment. These limitations are inherent in case-control studies. Quality of evidence can be improved by conducting prospective cohort studies.