An industry is a group of companies that are related based on their primary business activities. In modern economies, there are dozens of industry classifications. Industry classifications are typically grouped into larger categories called sectors.

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The most common methods include the Standard Industrial Classification (SIC) system and the North American Industry Classification System (NAICS). The SIC system was developed in the 1930s and is based on the production-oriented approach, while the NAICS system was developed in the 1990s and is based on the supply-oriented approach.

Another way to classify industries is through the use of financial ratios, such as the price-to-earnings ratio (P/E ratio) or the return on equity (ROE). This method involves comparing the financial performance of companies within the same industry to determine which ones are performing better.

Compustat, a database of financial information on publicly traded companies, can also be used for industry classification. Compustat assigns a primary industry code to each company based on its primary business activity. This code can be used to classify companies into different industries.

Other methods of industry classification include the Global Industry Classification Standard (GICS), the Industry Classification Benchmark (ICB), and the Financial Times Stock Exchange (FTSE) All-World Index.