First we justify our concern in latency constraints for real-time systems with precedence constraints. We evoke the model based on graph theory used to state and solve the problem of non-preemptive scheduling systems with precedence and latency constraints. Because latency constraints involve pairs of operations related to the graph which models the precedence constraints, we define three relations between pairs of operations, relations which determine the way of scheduling the operations. We give a necessary and sufficient condition of schedulability for the particular case of systems with precedence and latency constraints, where all pairs, on which latency constraints are imposed, are in one of these relations. Finally, we give a general schedulability condition for real-time systems with precedence and latency contraints and we propose a scheduling algorithm which is proved optimal.