An important component of the Common Core State Standards for Mathematics (CCSSM), used by the majority of states in the U.S., has the eight standards for mathematical practice (SMPs). While surveys have investigated teachers' perceptions of the CCSSM few have investigated middle school mathematics teachers' (MSMTs') (grades 6-8) perceptions of the SMPs. Similar to other countries, teachers in the United States frequently use mathematics textbooks. Two different types of textbooks have been in use in the United States, conventional and standards-based. The latter were designed on the basis of earlier standards documents including versions of the SMPs. As a result of these changes to the educational system in the U.S., we sought to characterize MSMTs' perceptions of the SMPs and investigate if these perceptions differed by the type of textbook teachers used. We found that MSMTs struggled in naming the SMPs, misinterpreted the modeling SMP, and conflated components of reform-oriented instruction with the SMPs. MSMTs using a standards-based textbook were more likely to view their textbooks as aligned with the SMPs, x[superscript 2](3) = 7.708, p = 0.026 and to view the SMPs as an instructional philosophy for the CCSSM, Fisher's Exact Test = 3.881, p = 0.05. The implications of these results are discussed.