Teachers are accustomed to gathering and analyzing data that show what a student knows, has learned, and has not yet learned. With this information, aggregated for the class, the teacher may alter her lesson plans to accelerate her pace or slow down to reteach. She may make notes to alter the lesson design for the next time she teaches the lesson. A teacher intent upon personalizing her instruction uses the data to adjust the assignments, supports, and expectations for each student based on that student's individual data. But without appropriate measures and careful analysis, data that show what a student knows do not tell much about how a student learns. To build more capable learners, the teacher must examine her students' learning habits and know how they learn and why they invest effort in learning. The learning challenge (goal, objective, task) is to achieve mastery of specified knowledge and skill under given conditions. To gauge the learner's capacity to achieve mastery, the strength of the learning habits is key, and the learning habits' strength is derived from the depth and vitality of the personal competencies and their repertoires. Fluency comes from the relative ease and quickness--the automaticity--upon which learning habits are exercised. In this brief, when considering of learning habits, the authors stress that the traces of how the student learns are also important as the teacher personalizes learning and creates opportunities for each student to personalize her or his own learning. [This topic brief is one in a series on personalized learning prepared for Conversations with Innovators, 2018.]