To make an HCCI engine as a useful commercial product, the engine has to be capable of performing quick transients in a large operating range, especially in vehicle applications. HCCI combustion is kinetically controlled and has to be operated properly between two limits: misfire and knock. To achieve the correct state, the right amount of fuel/air/EGR has to be inducted into the cylinder. The amounts and ratios of the three components are highly dependent on other variables as operating conditions change. It is unrealistic and unreliable to predict the right combination of these variables without principal component analysis. Thus, the optimal response control path has to be based on the quality of the previous combustion event as well as the direction and the rate of transition.The MRPR (Maximum Rate of Pressure Rise), when used between misfire and knock limit, is the key measure which carries, qualitatively and quantitatively, the most valuable information about an HCCI combustion event. While MRPR is directly proportional to the fueling rate on a cycle to cycle basis, the sensitivity of MRPR to fueling change is indicative of whether the ratio of fuel/air/EGR is proper at a given engine operating condition. Including MRPR in the control state matrix helps to define and extend healthy combustion state space and thus achieve optimal transient controllability. Since cylinder pressure can be measured and thus MRPR calculated, it can be used as feedback to provide corrective action for the next cycle both in direction and quantity.In this paper, the theory behind this concept will be explored. Experimental validation of using MRPR for reliable transient control with an actual vehicle will be presented, including driving cycle tests and road tests. Methods for building MRPR physical effect observers to control HCCI transient operation are also studied. Using proxies of MRPR from extra-cylinder signal sources such as knock sensors and crankshaft dynamics would improve reliability and reduce cost thus aiding in bringing HCCI applications to market.