The forensic potential of microorganisms is becoming increasingly apparent as a consequence of advances in molecular sciences and genomics. This review discusses instances in which microbes, and in particular bacteria, can impact upon forensic investigations. There is increasing evidence that humans have an extremely diverse ‘microbiome’ that may prove useful in determining ethnicity, country of origin, and even personal identity. The human microbiome differs between regions of the body and may prove useful for determining the nature of stains such as those caused by saliva and vaginal fluid: it may even be possible to link the stains to the person responsible for them. Similarly, the composition of the microbiome present in a soil sample may prove a useful indicator of geographic origin or as a means of linking people, animals, or objects together or to a specific location. Microorganisms are important in the decay process and also influence the presence and concentration of alcohol, drugs, and other chemicals of forensic relevance. There is also a possibility that the entry of microorganisms into the body during the agonal period may prove useful for the diagnosis of drowning. The transmission of infectious diseases, and in particular sexually-transmitted diseases, can provide evidence linking a victim and a suspect. Microorganisms that cause fatal infections are not always identified at the time of death and may lead to the death being considered ‘suspicious’. If a fatal infection can be linked to a hospital or medical procedure it can lead to prosecutions and therefore it is important to determine when and where an infection was acquired. Similarly, naturally acquired infections need to be distinguished from those that result from malicious transmission. Microorganisms can therefore provide evidence in many different forensic scenarios but most of the work is still at the experimental stage and there are therefore many opportunities for further research.