Microalloyed (MA) steels have been developed as one of the most significant metallurgical advances over the last thirty years, with their property improvement and cost effectiveness characteristics. Even though the underlying principles for microstructural property control of these steels have been well established, applications of these steels are still limited in scale mainly due to a lack of their understanding. This review paper focuses on mechanical property control of these steels. Since the properties depend mainly on the composition and microstructure which in turn are controlled by steel making and processing, metallurgical variables are reviewed in this first part of the review. These include their strengthening mechanisms, effects of composition and processing on their behavior, and the various MA steel microstructures.