This paper presents optimal symbol constellations for M-PAM with energy detection receiver in fading channels. A significant gain in the symbol error probability can be achieved for medium to large degrees of freedom. We also present closed-form solutions for the symbol error probability in flat, approximated frequency selective and frequency selective fading channels. Independent and correlated fading has been considered. The assumption of Gaussian probability density functions based on the central limited theorem has not been used, because of the inaccuracy for low and medium degrees of freedom.