Purpose of reviewOnline monitoring of haemodialysis provides more detailed and more immediate measurement of parameters currently assessed during haemodialysis. It can also assess novel variables that are not routinely measured and could intensively monitor the haemodynamic response to dialysis. In an era of expanding numbers of increasingly dependent patients on dialysis, the ability to provide intensive and extensive monitoring of haemodialysis is a boon. The technology available to do this is rapidly becoming more sophisticated, more widely available and more diverse than ever before.Recent findingsAdequacy of urea removal is of crucial importance to mortality on haemodialysis, and online monitoring of urea removal is now achievable using a number of different methods. Some of these methods also measure sodium flux, allowing precise monitoring of sodium balance. Haemodynamic disturbance can be assessed using relative blood volume monitoring equipment, or more directly with non-invasive pulse-wave analysis giving a continuous blood pressure reading. The attributes of each of the separate technologies involved are examined.SummaryOnline monitoring of a variety of pertinent parameters during haemodialysis provides an array of tools that are steadily being incorporated into routine clinical practice. Research is also benefiting from novel insights into the process of haemodialysis provided by this technology.