Does a company have to pay a promised bonus to your salary if you have met all the requirements Or can they just decide not to pay any bonuses at all even though they were promised in writing?

If it is as straight forward as you say, that there is a written agreement between the employer and employee, to pay an amount of money conditioned only on the employees performance, and that performance is achieved....then payment can probably be enforced by law, or the State authority there that controls such. A common example would be something like "if you remain an employee for 12 months after signing on, an additional bonus of X will be paid on the next pay date". However, many bonuses are based on performance or actions other than the employees. Like certain measures in the Company...sales, profits, new customers, etc., all of which may not have been or even could have been met. Many also have fairly general statements, inserted by lawyers, about how the bonus is intended but voluntary on the part of the company and only payable if at all by it's own discretion. Obviously a problem in claiming it is due you.