Response to Intervention (RTI) models have attracted great attention as an alternative to traditional methods used for identifying students with learning disabilities. A major feature of this approach is the implementation of academic interventions in general education and measuring the student's response to those interventions. A common question that needs to be addressed is the number of stages or tiers of intervention necessary in the Response to Intervention model. This article reviews three studies of RTI that investigate the RTI tiers. Sharon Vaughn and Rollanda O'Connor report on studies using the Standard Protocol approach to RTI. David Tilly reports on using the Problem Solving Model as an RTI model. This article summarizes the results of these three investigations.