A fuller discussion of the databases listed below and procedures for assembling trade can be found on the  Subject Guide: Trade Data.  

UN Comtrade:  a comprehensive trade database derived from the international trade statistics reported to the United Nations by over 100 countries.  Quantity data is often provided in addition to the value in U.S. dollars.

Standard International Trade Classification (SITC): maintained by the Statistics Division of the United Nations, the SITC is the most commonly used schedule for categorizing trade commodities for analysis. Other trade coding systems are explicated in the Subject Guide: Trade Data.

Harmonized System Codes (Harmonised System or HS): another commodity code system used by the UN (e.g., in UN Comtrade) and by many national governments.

World Trade Organization:  the WTO website offers an interactive statistics database as well as country profiles. Much of the data is taken from the annual WTO publication International Trade Statistics, available from the website in pdf.

International Trade Administration:  a unit of the U.S. Dept. of Commerce which can be a prime source for statistical information on the international economy, especially the TradeStats Express data service.

United States International Trade Commission: source for U.S. trade data. Go to the Interactive Tariff and Trade DataWeb for international trade statistics and U.S. tariff data.  Registration is required, but free.  Consult the Guide for instructions on use.