Laravel validates incoming data by applying rule definitions to request fields. A controller can call $request->validate([...]), use a Form Request, or create a validator manually with the Validator facade.

When validation passes, the application receives the validated data. On a traditional web request, validation failure redirects back with errors and old input; on an XHR or API request, Laravel returns a 422 response containing validation errors. Rules can cover required fields, types, lengths, formats, database uniqueness, files, arrays, and conditional requirements.