We present here a broad overview of the field of metabolic engineering, describing in the first section the key fundamental principles that define and distinguish it, as well as the technological and intellectual developments over the past approximately 20 years that have led to the current state of the art. Discussion of concepts such as metabolic flux analysis, metabolic control analysis, and rational and combinatorial methods is facilitated by illustrative examples of their application drawn from the extensive metabolic engineering literature. In the second section, we present some of the rapidly emerging technologies that we think will play pivotal roles in the continued growth of the field, from improving production metrics to expanding the range of attainable compounds.