Renal replacement therapy in the intensive care unit can vary from simple procedures to very complex technologies. I discuss the factors that contribute to the decisions regarding the selection of a specific therapy. These factors include immediate and intermediate therapeutic goals (e.g., solute removal, dehydration), hemodynamic stability and other clinical conditions, and the technical requirements obligate to specific therapies. The differences between convective and diffusive solute removal are described, as well as the rationale for choosing one over the other. This choice is particularly relevant to and dependent on the therapeutic goal. Spontaneous blood flow versus pumped blood flow is debated. Lastly, the removal of middle molecular weight molecules is discussed in the context of toxic cytokines or bacterial products.