An unlawful detainer is when someone is occupying or possessing your property and they are not a tenant. For instance, you may have invited this person to stay in your home temporarily and now you’ll find that they don’t want to leave. The police aren’t really equipped to handle that situation so it must go through the civil court system. An unlawful detainer is similar to an eviction, but the difference is the person is not a tenant. They’re not paying rent. They’re just refusing to leave your property.