Nonlinear effects in the cosmic microwave background

Major advances in the observation and theory of cosmic microwave background anisotropies have opened up a new era in cosmology. This has encouraged th e hope that the fundamental parameters of cosmology will be determined to h igh accuracy in the near future. However, this optimism should not obscure the ongoing need for theoretical developments that go beyond the highly suc cessful but simplified standard model. Such developments include improvemen ts in observational modelling (e.g. foregrounds, non-Gaussian features), ex tensions and alternatives to the simplest inflationary paradigm (e.g. non-a diabatic effects, defects), and investigation of nonlinear effects. In addi tion to well known nonlinear effects such as the Rees-Sciama and Ostriker-V ishniac effects, further nonlinear effects have recently been identified. T hese include a Rees-Sciama-type tensor effect, time-delay effects of scalar and tensor lensing, nonlinear Thomson scattering effects and a nonlinear s hear effect. Some of the nonlinear effects and their potential implications are discussed.