Optical transmission measurements are commonly used for the routine determination of thin film optical constants. This paper presents an overview of the different methods of evaluating these transmission data, leading to values for the complex refractive index. Three different groups of methods are distinguished using: (1) at least two different optical measurements; (2) dispersion relations or general physical constraints to approximate the behaviour of the wavelength-dependent refractive index; and (3) a `virtual' measurement as a second variable. The methods from groups (2) and (3) (requiring only a single transmission measurement) are treated in more detail and are evaluated in terms of their accuracy.