An evaluation is given of the methods of modeling concentration profiles of suspended non-cohesive sediments in natural streams and under waves. The bottom boundary condition and the distribution models are treated separately. The merits of the simple concentration boundary condition, which has been favored by practice, are compared to those of the more general pickup function approach. Experimental evidence is presented for the inadequacy of pure gradient diffusion as a framework for modeling sediment suspension in streams and under waves. But problems arise when a significant part of the upward sediment flux is carried by vortices through distances comparable with the overall vertical scale. The performance of an alternative distribution model is discussed.