For decades, McKinsey has counseled some of the world's most successful companies. The lion's share of work, up to 40%, centers on strategy. But consulting on operations, organizational issues, and marketing also are big chunks--between 10% and 20%.HEWLETT-PACKARD

A key adviser to CEO Carly Fiorina on HP's recent acquisition of CompaqJOHNSON & JOHNSON

A longtime, highly successful client that has used McKinsey on dozens of engagements over 30 yearsGENERAL MOTORS

Continues to assist the big auto maker, which has shown gains over the competition in the past yearSIEMENS

Longtime counselor to vast German conglomerate on everything from sourcing parts from low-cost nations to factory consolidationHOME DEPOT

Advised the big-box retailer on its expansion into Latin AmericaAEROFLOT

Assisted in the turnaround of Russia's biggest airline

Data: BusinessWeek