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Generally, anytime you send out your resume. Never send your resume out without a cover letter. Even when employers don't specifically ask for a cover letter in their ads, they expect one. The exception is the occasional ad that states "resumes only." That's usually a tip-off that the company is expecting a large volume of responses and plans some sort of major screening of the many resumes that come in. These days, the screening device is likely to be the electronic scanning of resumes, in which computers are programmed to screen out resumes that don't contain the exact specifications the employer seeks. Sometimes, however, if you make the cut after resumes are winnowed down to a manageable level, the employer may contact you and ask you to send a cover letter at that point.