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In rapidly rotating stars terms quadratic and of higher order in Ω can no longer be ignored. This also includes the distortion of the star by rotation (see Section 3.2.4.2). For moderate rotation a perturbation expansion includ- ing terms of order Ω2 can be carried out. This was initially discussed by Simon (1969) and further developed, e.g., by Chlebowski (1978), Saio (1981), Gough & Thompson (1990) and Dziembowski & Goode (1992).32 The treatment was extended to third order in Ω by Soufi et al. (1998; see also Karami 2008). An interesting analysis to O(Ω2) was carried out by Sobouti (1980), consid- ering also an expansion around the convectively neutral state. Christensen- Dalsgaard & Dziembowski (2000) summarized the main aspects of the pertur- bation treatment of rapid rotation; the present section draws heavily on their presentation. For simplicity we assume uniform rotation in the following.