A P45 is a tax document issued in the United Kingdom when an employee leaves a job. It provides important information about the employee's earnings and the taxes that have been deducted during their employment. The P45 is divided into several parts, which the employee and their new employer will use for tax purposes. When starting a new job, the employee must give their P45 to their new employer. This helps ensure that the correct amount of tax is deducted from their salary. If an employee does not have a P45, they may need to fill out a Starter Checklist to determine their tax code.