The main objective of this monograph is to develop efficient techniques for tackling the control problems of partially-known dynamical systems. This broad class of systems has a fundamental feature: Knowledge of the dynamic characteristics of the control plant is not complete. In one category, the missing knowledge is due to the unmodeled high-frequency parasitics (adaptive systems). In another category, some of the model parameters are unknown-but-bounded (uncertain systems). The book concentrates on reduced-order adaptive control schemes and deterministic stabilizing control methodologies. Special emphasis has been placed on presenting new results, establishing efficient control laws and treating both small-scale and large-scale problems. Effects of external disturbances, state parameter errors and interconnection patterns have been investigated and dealt with in contructing various controller structures.