Who owns a patent when an employee invents it?

It depends on the country and on the contract, and the default is not always the employer. In the United States, the inventor owns the invention unless there is an assignment; an employer gets ownership through a written invention assignment agreement or, absent one, only if the employee was specifically hired to invent that thing. Otherwise the employer may hold a "shop right", a non-exclusive licence to use the invention, while the employee keeps title. Several states, including California under Labor Code section 2870, void clauses that claim inventions made on the employee's own time without company resources and unrelated to the business. In the United Kingdom, section 39 of the Patents Act 1977 gives the employer inventions made in the course of the employee's normal or specifically assigned duties, where an invention might reasonably be expected, and inventions by senior employees with a special obligation to further the business; everything else belongs to the employee, and under section 40 an employee can claim compensation where a patent has been of outstanding benefit to the employer.

Germany's Employee Inventions Act requires the employee to report every service invention in writing; the employer is deemed to have claimed it unless it releases the invention within four months, and must pay the inventor statutory compensation. Sweden's Act on the Right to Employees' Inventions gives the employer rights to inventions within the scope of the employee's duties in exchange for reasonable compensation, with collective agreements often filling in the detail; university teachers and researchers in Sweden keep their inventions under the teacher's exemption. The practical rule for a company in any of these countries is the same: do not rely on the default. A written assignment signed at hiring, a disclosure process that captures inventions as they happen, and, where the law requires it, a compensation policy are what make the company's ownership hold up in due diligence. This is general information, not advice on a specific contract.