Most folks don't bump up against this topic until they start their first business.  By law, every state requires that each business entity has a registered agent if that business entity "does business" in their state.

Registered agents, also known as resident agents or statutory agents, have many functions but the primary one is to receive "service of process."  Which is a legal term for somebody that is available to get “served," and not in a dance-off kind of way, but in a you're getting sued kind of way.  

Often, a small business will use one of its own people as its registered agent in the state where it is located so long as it has a street address and not just a PO box.  

States require that the registered agent is personally available during business hours.  

However, if a company (which includes sole-proprietors) does business in an additional state, or states, the company will need to hire a registered agent in the other state, or states.  Unless, of course, that company has another office in the additional state and has appointed an in-house registered agent at their street address that also maintains regular business hours. 

So even if someone is able to act as her own registered agent in her own state, she will need to hire a registered agent in the other states where she is also “doing business.”

You can read up on what “doing business” means in another post but keep in mind the bar is really low.  As you will read in our post on "doing business", if you even have to ask the question, the state you are wondering about will likely classify your business activity as "doing business" in their state.