This paper reviews the optical techniques for in-cylinder combustion temperature measurement, particularly soot measurements in diesel engines. The review starts with the two-colour method for in-cylinder soot and combustion temperature measurement. The principle and implementation of the two-colour technique are described in detail. Both signal point and full-field temperature and soot measurements by the two-colour method are considered. In the second part, the soot diagnostics based on light scattering, especially the light extinction method for in-cylinder soot concentration measurements, are discussed. In the third part, optical techniques for spatially resolved two-dimensional measurements of soot particles in diesel engines are introduced. Since laser induced incandescence (LII) is a relatively new technique and is particularly suitable for the two-dimensional imaging of soot distribution, the operating principle and implementation of LII are discussed in detail. At the end of each part, examples are given to illustrate the understanding gained about diesel combustion as a result of the application of these optical techniques. This paper provides a comprehensive review for those who are interested in using optical diagnostics for in-cylinder soot and combustion temperature measurement in diesel engines.