This chapter discusses the most important characteristics of fluorescence that plays a fundamental role in understanding the basics and the applications of Förster (fluorescence) resonance (radiationless) energy transfer (FRET). FRET is the transfer of electronic excitation energy between isolated donor D and acceptor A of suitable spectroscopic properties. The donor molecules, typically, emit at shorter wavelengths, which overlap with the absorption spectrum of the acceptor. This energy transfer occurs without the appearance of the photon and is the result of long-range interactions between the D and A dipoles. The most important factors affecting FRET are the overlap integral, the quantum yield of the donor in the absence of the acceptor, and the orientation factor. The quantitative analysis of steady-state and time-resolved FRET measurements provides information on global structures and conformational dynamics, and reveals thermodynamic parameters for conformational transition. This information is essential for the understanding of biological functions of proteins, DNA/RNA, and other biological assemblies that are frequently mediated by transitions between alternative conformations.