Despite the advances made in risk assessment i the past twenty years, in areas as diverse as toxicology and offshore engineering, the risk assessment approach has made little impact on those addressing the microbiological aspects of public health. In this paper the advances which have been made are discussed and the difficulties preventing the wider application of microbiological risk assessment (MRA) to public health are considered. The term microbiological risk is used here to mean the probability of contracting a disease caused by a microorganism. I intend to demonstrate that the dynamic nature of microorganisms and the unique nature of the relationship between a pathogen (a microorganism which causes disease) and its host create special challenges for those involved in MRA. Although these problems are difficult they are not intractable. Indeed in some cases partial solutions have already been found and applied. It is hoped that this paper will help stimulate further thought and consideration in a variety of disciplines so that these challenges can be met, thereby allowing MRA to fulfil its potential.