I agree that it is important to complete Mathematical Thinking at First Grade first. In New Castle County, we have an organization called "Math Quest," that holds "Investigations..." trainings, reunions meetings, and TERC 2 sessions. The reunion meetings and TERC 2 sessions provide us with the opportunity to discuss and share our concerns and successes with the curriculum. This topic of whether or not to do "Mathematical Thinking at..." has come up more than once. All of the teachers seem to agree that at grades 1 - 3, at least, this book is very necessary for a successful year with "Investigations..." It helps to set up the mathematical environment and establish the routines for the year, as well as provide the teacher with a snap shot of where his/her students are in their understanding of mathematics.