One of the bedrock principles of worker's compensation is universal coverage. That means that virtually every employee is covered. Therefore, virtually every employer has to have worker's compensation insurance. Specifically, the law defines the following as employers who must have worker's compensation insurance:
Universal coverage really means universal. Even out-of-state employers with employees working in Wisconsin must have a worker's compensation policy with an insurance company licensed to write worker's compensation insurance in Wisconsin. If an out-of-state employer has a worker's compensation insurance policy with an insurance company not licensed to write in Wisconsin, they must obtain a policy from a Wisconsin licensed insurance company.
Universal coverage also applies to those employers who are financially sound (and usually quite large) that are "self-insured". Being Self-insured means that the employer is not required to obtain worker's compensation insurance from an insurance company because they can bear the financial obligations of paying claims from their own internal resources. The Department of Workforce Development grants permission for employers to become self-insured.