Within our domain of interest, images are formed by light and its interaction with matter. The spatial and spectral distribution of light is focused on the sensor and recorded as an image. It is therefore important for us to first understand the nature and the properties of light. After a brief description of the nature of light, we will discuss some of its basic properties: energy, frequency, coherence, and polarization. The energy flow of light and the characterization of the frequency/wavelength distribution are the subjects of radiometry, colorimetry, and photometry, which will be covered in later chapters. The coherence and the polarization properties of light are also essential for understanding many aspects of the image formation process, but they are not as important for most color imaging applications because most natural light sources are incoherent and unpolarized, and most imaging sensors (including our eyes) are not sensitive to polarization. Therefore, we will discuss these two properties only briefly. They are presented in this chapter. Fortunately there are excellent books [208, 631, 871] covering these two topics (also, see the bibliography in Handbook of Optics [84]). From time to time later in the book, we will need to use the concepts we develop here to help us understand some of the more subtle issues in light matter interaction (such as scattering and interference), and in the image formation process (such as the OTFs).