Deloitte, PricewaterhouseCoopers (PwC), KPMG, and Ernst & Young (EY)—are the largest professional services firms, dominating the accounting, audit, tax, and advisory sectors. While they began as accounting firms, they have expanded into management consulting, legal services, and corporate finance, serving clients across various industries. One key differentiator of Big 4 consultancies is that they mostly focus on advising companies on executing plans that have already been set into place (i.e. integration consulting), and typically have smaller dedicated branches to strategy, like "Monitor Deloitte", "Strategy& by PwC", and "EY-Parthenon", which have developed through means of acquisition. Collectively, Big 4 firms hold over 35% of the consulting market share, making them key players in the global business ecosystem.

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