Every school district has policies and procedures for evaluating teachers that are shaped by state law and local collective bargaining agreements. As a result of No Child Left Behind and other accountability legislation many states modified requirements about the frequency of evaluation and now require the use of student achievement data as one measure of effectiveness. The research is clear that effective evaluation systems are based on a mutually agreed upon model of good teaching. Effective systems include both a formative and summative component and provide for self-assessment and reflection. The most effective models reflect a culture of collaboration and commit to continued professional growth. They use multiple measures and recognize that no single visit to a classroom can provide a comprehensive understanding of a teacher's knowledge and skill. In recent years principals have begun to use mobile technology such as iPads and laptops and software designed purposively to record data gathered during classroom visits. Many different vendors provide such software, some tailored to the individual needs of school districts.