Let us start by saying that mechanical ventilation is not a benign therapy. It is not a beautiful universally tolerated life-saving solution to any sort of respiratory failure. It is a crude and ugly mechanical torture. Imagine being captured by a stranger, drugged, tied to a bed and forcefully insufflated with pressurised gas. Needless to say, we reserve such treatment only for situations where it can be reasonably expected to have some positive effect, and where the alternative is death or (worse) survival with disability. This chapter discusses the indications for mechanical ventilation, as well as situations where it may be inappropriate. It is important for the intensivist to be intimately familiar with the latter subject area; one needs to be able to justify the reason for withholding lifesaving treatment beyond "they're old and frail" or "the COPD is too severe".