The performance of a counterflow, rotary heat exchanger operating with either transient or nonuniform inlet temperatures is investigated. The effect of transient inlet temperatures is analyzed in terms of the response of the outlet fluid temperatures to a step change in temperature of one of the inlet fluid streams. The effect of temperature nonuniformities is analyzed in terms of the change in steady-state effectiveness due to a circumferential temperature distribution in one of the inlet fluid streams. These temporal and spatial variations are explored using three different methods of analysis. An equilibrium analysis, assuming infinite heat transfer coefficients, is developed from kinematic wave theory. It is used to qualitatively describe the heat transfer process and define the upper limit of performance. A finite difference model of the governing differential equations, using finite transfer coefficients, is employed to obtain a detailed numerical analysis of heat exchanger performance. Results for the complete range of matrix to fluid capacity rate ratio are presented for a balanced and symmetric regenerator. At moderate capacity rate ratios, the numerical analysis predicts unusual temporal periodicity in the transient response. An experimental analysis has also been conducted using a counterflow, parallel passage, rotary heat exchanger made from polyester film. The results are used to substantiate predictions of the numerical model.