A formula for calculating the number of civilizations in our Galaxy from which we might detect artificial transmissions. The equation, proposed in 1961 by the American radio astronomer Frank Donald Drake (1930– ), multiplies together the following factors: R*, the rate of formation of stars in our Galaxy with suitable environments for the development of intelligent life; f_p, the fraction of those stars that have planetary systems; n_e, the number of planets in each system that are suitable for life; f_l, the fraction of such planets on which life actually arises; f_i, the fraction of life-bearing planets on which intelligent life emerges; f_c, the fraction of civilizations that release detectable signs of their existence into space; and L, the average lifetime of such civilizations. The solution to the equation depends on the values assigned to each factor, many of which are not well determined. Possible results for the number of communicative civilizations in the Galaxy at present range from almost zero to many thousands.