||In this talk I will address the major issues in calculating an adequate sample size for cluster randomised trials. It has long been recognised that in order for these trials to be adequately powered, between cluster variability must be accounted for in the sample size calculations. This is usually done by using an estimate of the intra-cluster correlation coefficient (ICC) in a design effect which is then used to adjust the sample size required for an individually randomised trial aiming to detect the same clinically important difference. More recently it has been recognised that variable cluster size should also be accounted for and a simple adjustment to the design effect provides a means to do this. Investigators still face three challenges, however: lack of information about variability in cluster size prior to the trial, lack of information about the value of the ICC prior to the trial, the adjustment for variable cluster size does not strictly match all methods of analysis. I will illustrate these challenges with some examples and outline approaches that have been and could be adopted to address them.