The human microbiome is being rapidly revealed to us through the power of next generation sequencing and bioinformatics. We are experiencing an explosion in data, information and hopefully knowledge about the role the microbiome plays in human health and disease. The average person harbours about 100 trillion symbiotic microbes with 95% of them residing in our gastrointestinal tract. Forty per cent of the dried weight of human faeces is microbial. The ratio of human cells to microbial cells is about 150:1 but because microbial size is on the scale of a mitochondrion we have not appreciated the number and complexity of our fellow travellers until now. For each bacterium there are about 2 bacteriophages (viruses) and it has been suggested that each bacterium is killed on average every 2 days by lytic phages. Probably of most interest to oncology is evidence that the microbiome of an individual tends to be very stable over time (apart from a short period of variation at the extremes of life) and that 95% of the genetic variation between individual humans resides in differences in microbial DNA, with only 5% related to human DNA. During this presentation I will introduce the microbiome, revise how we detect differences within and between individuals and review evidence that relates our microbiomes to cancer susceptibility, risk, response to cancer therapy and the potential role of microbiome screening for the early detection of colorectal cancer.