In this paper, we examine some principles in managing manufacturing systems. These principles are concerned with the variability, the utilization, the rework, the lead time, and the constant work-in-process efficiency. While these principles are developed through analyzing some simpler disconnected flow line manufacturing systems, we examine whether they can have broad applications. For some of these principles, we provide sufficient conditions, while for others, we provide counterexamples. Our analysis suggests that we should be very cautious about these laws when applied to non-Markov and non-tandem systems.