If you are paid by foreigners then it is quite possible they don't file anything with the IRS.
All of this income you are required to report as business income on schedule C. There are opportunities on schedule C to deduct expenses like your health insurance, travel, telephone calls, capital expenses like a new computer, etc... You will be charged both the employees and employers share of social security/medicare, around ~17% or so, and that will be added onto your 1040.
You may still need a local business license to do the work locally, and may require a home business permit in some cities. In some places, cities subscribe to data services based on your IRS tax return.... and will find out a year or two later that someone is running an unlicensed business. This could result in a fine, or perhaps just a nice letter from the city attorneys office that it would be a good time to get the right licenses.
Generally, tax treaties exist to avoid or limit double taxation. For instance, if you travel to Norway to give a report and are paid during this time, the treaty would explain whether that is taxable in Norway. You can usually get a credit for taxes paid to foreign countries against your US taxes, which helps avoid paying double taxes in the USA.
If you were to go live in Norway for more than a year, the first $80,000/year or so is completely wiped off your US income. This does NOT apply if you live in the USA and are paid from Norway.
If you have a bank account overseas with more than $10,000 of value in it at any time during the year, you owe the US Government a FinCEN Form 114 (FBAR). This is pretty important, there are some large fines for not doing it. It could occur if you needed an account to get paid in Norway and then send the money here... If the Norwegian company wires the money to you from their account or sends a check in US$, and you don't have a foreign bank account, then this would not apply.