Optical diagnostics are between the most commonly used tools for the characterization of plasmas and for the better understanding of physical and chemical processes controlling their time and space dependencies. In this review paper the most popular spectroscopy techniques are described and examples of their applications to plasmas are indicated. An special emphasis is given to power saturation phenomena, encountered in laser based diagnostics. Numerical applications show that with commonly available lasers a great care must be taken to avoid saturation and to be really non-intrusive, as expected for a diagnostic tool.