Although almost every business is unique in its own way, there are some universal lessons that apply to just about any kind of business. In these case studies, we'll take a look at employee performance and retention, supply chain management, growth, ad spending, and more. Although the following are focused on specific businesses, all business students can learn lessons from their triumphs and mistakes. Read on, and you'll find 20 classic case studies you'd do well to know as a business student.