Ignition delay times for conventional and alternative jet and diesel fuels were measured in a constant volume spray combustion chamber and a shock tube in homogenous gas-phase reflected shock experiments. Experiments were performed in the spray environment for injection of liquid fuel sprays into hot air at 1.0, 2.14, and 4.0 MPa and 620-830 K. Shock tube experiments were performed for homogenous stoichiometric fuel/air mixtures at pressures around 2, 4, and 8 MPa and for 660-1310 K. These experiments characterize relative fuel reactivity, the dependence of reactivity on temperature and pressure, and allow for correlation of reactivity under spray and homogenous gas-phase conditions. Important results include the observation of three temperature regimes for reactivity in the shock tube experiments: high-and low-temperature regimes with positive overall activation energy connected by a negative-temperature-coefficient (NTC) regime. While increasing pressure was shown to result in decreasing ignition delay in all regimes, pressure was shown to have an appreciably larger influence in the NTC regime. Spray ignition experiments exhibited decreasing temperature dependence with increasing temperature from the low-temperature regime towards the entrance to the NTC. Fuel reactivity trends from spray ignition experiments, quantified using the derived cetane number (DCN), were found to correlate via a power law relationship with ignition delay times measured in the shock tube at NTC conditions, suggesting that DCN is primarily sensitive to NTC gas-phase chemical kinetic reactivity. The extensive experimental database reported here for jet and diesel fuels at high-pressure engine-like combustion conditions should be valuable for the future development of real fuel chemical kinetic modeling.