Registered Apprenticeship for young people is an earn-and-learn program for women and men in their late teens and early twenties. It combines academics with paid work experience across many sectors. An essential part of the apprenticeship experience is to be paired with a mentor, who supports, advises, and instructs an apprentice(s) on the job. The quality of mentoring is critical to apprentices' learning and persistence in the program and is a major factor in program delivery success. This first edition of the Urban Institute's mentor guide has been developed for experienced workers who teach and advise youth apprentices at work. It provides a variety of tips for mentors, guidance on common mentoring topics, frequently asked questions, and links to resources. As a tool, it acts as a handy reference guide and is intended to be paired with other learning and/or preparation for mentors in executing effective apprenticeship programs. [This report was written with Zach Boren, Bhavani Arabandi, and Tamar Jacoby.]