What are the differences between Systematic Desensitisation and Flooding?

One difference between Systematic Desensitisation (SD) and Flooding as treatments of phobic anxiety is the time period used. SD is a behavioural therapy based on the principles of classical conditioning, designed to reduce the phobic anxiety gradually, over time. In comparison, Flooding involves sessions of around 2-3 hours where the patient is faced with their phobic stimulus immediatley until their anxiety decreases. A further difference is in terms of costs. Flooding is seen as more cost effective because often only one session is needed to cure the patient of their phobia, whereas SD will take place often multiple sessions, as the key aspect is that the phobia is reduced gradually. An additional difference is based on ethics: Flooding is seen as very unethical because of the intense psychological distress it causes, whereas SD is a calmer and more appropriate technique.