The classical group sequential procedure is probably the most commonly applied method to facilitate the conduct of interim analysis in clinical trials, especially trials with mortality or severe or irreversible morbidity as the primary endpoints. The International Conference on Harmonization Guidelines for Industry discuss regulatory requirements on interim analyses in general and group sequential procedures in particular, regarding data monitoring and early stopping. All interim analyses should be carefully planned in advance and fully described in the protocol. Unplanned interim analyses should be avoided if at all possible. In the event that an unplanned interim analysis was conducted, a protocol amendment describing the interim analysis should be completed prior to unblinded access to treatment comparison data. Any interim analysis planned for administrative purposes only should also be described in the protocol and subsequently reported. Examples include speeding up recruitment and updating trial progress for stakeholders.