Marsh McLennan is a professional services firm built around insurance brokerage, risk management consulting, human resources consulting and investment advisory services, serving businesses that need help navigating complex risk and benefits decisions. Its Marsh division is one of the largest insurance brokers in the world, helping large companies design and place commercial insurance coverage, while Mercer and Oliver Wyman provide consulting on retirement plans, benefits and broader business strategy. As a broker and advisor rather than an insurer itself, Marsh McLennan doesn't carry underwriting risk on its own balance sheet, earning fees and commissions instead for the advice and placement services it provides. The firm has grown substantially through acquisitions over decades, consolidating smaller brokerages and consulting firms into its four main operating segments.