Epidemiology is a large and complex subject based on very simple principles. This topic looks at epidemiology and health control measures in populations. An epidemiologist looks for patterns and tries to determine what the difference is between affected and unaffected units. We are designed as humans to look for patterns in everything and do it all the time whether there is a pattern there or not. What epidemiology does is to formalise that process so that you can determine how confident you should be in your estimate of what is happening. The topic begins by looking at epidemiological principals, followed by a discussion of population sampling exercises such as sample size calculation and analysis of outbreak information. We then look at methods of developing a differential diagnosis, risk factors and means of controlling suspected disease outbreaks if they should occur. This topic will be of use to those interested in farm management and fish health alike.