In the last few years, there has been a very rapid increase in the number of applications of laser diagnostics in internal combustion (IC) engines, particularly the use of optical techniques for studying mixture preparation and in-cylinder mixture distribution. This is due to the importance of mixture formation in controlling the subsequent combustion and pollutant formation in IC engines. This paper attempts to review and discuss the current status of the applications of these optical techniques for in-cylinder mixture formation studies. The first half of this paper focuses on the three optical techniques used for the measurements of fuel vapour concentrations in IC engines. These are Laser Rayleigh Scattering (LRS), Spontaneous Raman Scattering (SRS), and Laser Induced Fluorescence (LIF). The second half of this paper is devoted to another two optical techniques, Laser Induced Exciplex Fluorescence (LIEF) and dual wavelength Laser Extinction/Absorption (LEA), which are capable of simultaneous measurements of fuel vapour and liquid droplets in IC engines. Simple theoretical descriptions are used to introduce the principle of each optical technique. Experimental considerations are given for the application of these techniques to IC engines. Much attention is devoted to providing readers with appropriate references to the technical literature, which will be of further assistance to readers interested in a particular application of each technique.