Free and mixed convection has been studied numerically for horizontal porous layers heated from below by multiple, isothermal, discrete sources. Steady-state results have been obtained for natural convection with up to eight sources and for mixed convection with up to four sources. Rayleigh numbers are varied from 10 to 500, and Peclet numbers from 0.1 to 100. For natural convection, the vertical sidewalls for the two-dimensional domain considered have little effect on Nusselt numbers if the distance between the outermost source and wall is greater than three times the length of the heat source. As the number of heat sources increases, heat-transfer coefficients for the second outermost sources are a minimum. In mixed convection, overall Nusselt numbers increase with both the number of sources and Rayleigh number. If the individual heat source is considered, heat-transfer coefficients are determined only by its location. For Rayleigh numbers greater than 50, there exists a critical Peclet number for which the overall Nusselt number is a minimum. Unstable, oscillatory flow and temperature fields are observed for Rayleigh numbers greater than 50.