Continuously increasing demand for energy resources drives petroleum industry to search in very deep waters, drilling very deep wells, encountering adverse conditions of pressures and temperatures and thus having to resolve a multitude of problems. In many cases, the problems can be attributed to the performance of drilling fluids which then presents challenges to drilling fluid industry. Such problems include poor hole cleaning, high pressure losses, loss circulation zones, fluid gelation, reservoir fluid invasions. Thus, drilling fluid industry must work hard to overcome these challenges figuring out solutions which include fluid formulations capable of good continuous performance under all adverse conditions. In this paper we address these issues, we present the adverse conditions in the deep drilling environment of pressures and temperatures, and the implications they have on flow pressures, formation damage and well control. We analyze the approaches taken by different operators in terms of drilling fluid design and implementation in such wells from a series of reported case studies. These include rheological measurements at high temperature and pressures and attempts to model-fit the data to predictive rheological models which could then be used in appropriate hydraulic models to estimate pressure drop profiles along the wellbore. High temperatures present additional stability challenges to drilling fluids which could be overcome with the addition of appropriate additives. The search for such additives is continuous and in this paper we discuss various alternatives, the operational procedures and address also potential problems. Finally, we present possible optimum solutions to overcome such problems and increase the success rate of drilling fluid performance in these difficult conditions.