Professional service firms (PSFs), like so many other companies, are juggling the modern challenges of global competition, increased regulation, and rapid employee turnover. In a people-oriented industry, attrition has special import. DeLong and Gabarro, of Harvard Business School, along with former Morgan Stanley and Ernst & Young executive Lees, argue that a PSF can gain a much-needed competitive edge by renewing its focus on mentoring. The authors' in-depth interviews with professionals from more than 30 PSFs have yielded four principles for firms to heed as they rediscover this lost art. First, mentoring is personal. Rather than relying on standardized programs, mentors must frequently--and fairly--provide authentic advice and nurturing. Partners at PSFs know how to use their ample people skills effectively with clients; the benefits of using them with junior colleagues are even greater. Second, not everyone is an A player. A small dose of attention given to a B player goes at least as far as a large one offered to an A player. Since B players constitute about 70% of PSF staff, that's time well spent. Third, choice assignments are in short supply, which limits the number of learning opportunities available for associates. Good alternatives include shadowing senior professionals on assignments and taking on research or other projects that are not client-related but that nonetheless build expertise. Finally, mentoring is a two-way street. Protégés should not only learn from their senior counterparts, but also be taught to attract mentors--and to co-mentor one another.