By agreement, states that have adopted the Common Core State Standards (CCSS) accept them as representing 85 percent of the total number of the standards in a subject area, meaning states have the option to identify as much as 15 percent in additional standards. Those that elect to add standards may find opportunities and hazards at each step--from the development of the new standards themselves to choices about how they will be addressed in curriculum, instruction, and assessment. For example, stakeholders who believe the CCSS standards could be improved upon will welcome the opportunity to address their concerns. But states will want to ensure the additional content represents meaningful and necessary expectations for all students, not a spreading and thinning of content that creates an unrealistic burden for teachers. The growing number of curriculum resources designed to support the Common Core will not address any unique state standards. Similarly, professional development programs developed around the Common Core will be found lacking when it comes to a state's special set. Assessment is the thorniest question of all. Who will assess these standards, how, and when? Despite these inherent challenges, a number of states have elected to develop "the 15 percent." It may be useful to identify a number of plausible scenarios to determine how states might make best use of the 15 percent option. Although how these play out may reflect a mix of the following approaches, it's useful to think about states as taking one of four major tracks: (1) Do nothing, accepting the Common Core as 100 percent of the curriculum by default; (2) Do nothing, but presume ownership and take advantage of the 15 percent of instructional time available; (3) Offer the choice of whether and how to use the 15 percent to districts; and (4) Adopt new statewide standards, taking full advantage of the 15 percent option.