The quantitative SECM theory was developed during the last decade for various regimes of measurements and electrochemical mechanisms. Different operating modes of the SECM, e.g., feedback and generation/collection modes, steady-state and transient measurements, require significantly different theoretical descriptions. Quantitative treatments are available for both diffusion-controlled processes and finite kinetics as well as for more complicated mechanisms involving adsorption and homogeneous reactions in the gap. The focus of this chapter is on fundamentals of the SECM theory, i.e., steady-state and transient measurements, feedback mode, and generation/ collection experiments. While the theory for a one-step heterogeneous reaction will be covered in detail, only a brief discussion of other processes (e.g., adsorption, charge transfers at the liquid/liquid interface) will be included to avoid overlap with other chapters.