Effective field theory is a useful tool to analyze non-perturbative physics. The effective Lagrangian is expanded in local operators that only involve the light degrees of freedom, where the expansion is in powers of the external momenta of the light fields divided by the scale of heavy physics. In principle it consists of an infinite sum of local operators which are typically of diminishing importance with higher dimension. In practice this series is truncated according to the accuracy desired for the process in question. With this Lagrangian one can then compute any low energy amplitude one wishes. I will explain what is an effective theory and how to construct it and how to use it using the example of Chiral Lagrangian and heavy quark effective theory.