The electrical interaction energy between two charged entities in an electrolyte solution plays a significant role in various phenomena in colloid and interface science. Available methods for the estimation of this energy under the Debye-Huckel condition are discussed briefly, and a systematic approach based on a boundary integral method, which has the potential to yield an approximate analytical expression for various types of surfaces under a general surface condition, is introduced. The linear sizes of the interacting entities can be comparable or one is much larger than the other. A typical example for the former includes, for instance, the interaction between two colloidal particles. The stability behavior of a colloidal dispersion belongs to this category. That for the latter includes the interaction between a particle and a wall. The adsorption of particles to surfaces and the electrophoretic motion of particles near a boundary, for example, belong to this category. Extensions to more complicated cases, for example, multiple particles and arbitrary surfaces, are also discussed.