A corporation conducting business in one state when incorporated in another is considered a foreign corporation and must qualify as a foreign corporation to legally do business in that state. For example, a business incorporated in Nevada that wants to do business in California would be considered a foreign corporation in California, and as a result, the Nevada corporation must qualify (or register, depending on the state) as a foreign corporation to do business in California.
What does "qualify" as a foreign corporation mean?
States generally have a similar filing process to qualify (or register) as a foreign corporation, but some states may require a bit more (please refer to the specific Secretary of State to get the detailed requirements for that state). However, here are the general requirements that almost all states will demand. The Secretary of State will require a foreign corporation to fill out a form, commonly called the "Foreign Corporation Certificate" or the "Statement and Designation by Foreign Corporation." This form will ask for the name of the corporation, the state of incorporation, the address of the principal office in the state of incorporation, the address of the principal office in the foreign state, and the name and address of the registered agent for that state. Every state will require a foreign corporation to provide the name and address of a registered agent in order to give the state a means to communicate with the foreign corporation. The registered agent (an individual or corporation) must generally reside within the state and provide a physical address (no P.O. Box). The form must be signed and dated by a corporate officer. Also note, there will be a filing fee for registering as a foreign corporation.
Many states also require a "Certificate of Good Standing"(or "Certificate of Authorization" or "Certificate of Existence" depending on the state) to be filed by the foreign corporation along with the form described above. This Certificate is used as evidence that the foreign corporation exists and is authorized to conduct business in that foreign state / country. This Certificate is issued by a state official from the state of incorporation.
Some states may require the foreign corporation to list its assets and liabilities as well as the assets and liabilities within the state it wishes to qualify to do business in. Also, some state may have stricter requirements regarding the registered agent.
DISCLAIMER #1 - Qualifying as a foreign corporation provides the state a means to tax the corporation for business it transacts within that state. This tax may be in addition to any other tax from the state of incorporation (essentially a possible double tax). Consult an accountant or attorney regarding the tax implications of conducting business in another state.
DISCLAIMER #2 - Not all transactions that occur within a state are considered "business transactions" in that state. Each state may have a unique interpretation of what is considered "business transactions" within that state and so certain businesses may not need to qualify or register with a particular state to do business in that state. For example, a Nevada corporation (e.g., Amazon) who merely solicits business through the internet and ships its products from Nevada to a customer in California most likely will not need to register the corporation in California even though millions of products are being sold to California customers. States are getting creative in taxing foreign corporations and are pushing the limits of what constitutes "business transactions" within a state, and so the definition of "business transactions" will be in flux and may be different depending on the state. For these reasons, it may be important to consult an attorney on whether a particular corporation will need to qualify or register as a foreign corporation. Penalties for not registering can be costly.