Vapor chambers can offer a passive heat spreading solution for thermal management in electronics applications ranging from mobile devices to high-power servers. The steady-state operation and performance of vapor chambers has been extensively explored. However, most electronic devices have inherently transient operational modes. For such applications, it is critical to understand the transient thermal response of vapor chamber heat spreaders and to benchmark their transient performance relative to the known behavior of metal heat spreaders. This study uses a low-cost, 3D, transient semi-analytical transport model to explore the transient thermal behavior of thin vapor chambers. We identify the three key mechanisms that govern the transient thermal response: (1) the total thermal capacity of the vapor chamber governs the rate of increase of the volume-averaged mean temperature; (2) the effective in plane diffusivity governs the time required for the spatial temperature profile to initially develop; and (3) the effective in-plane conductance of the vapor core governs the range of the spatial temperature variation, and by extension, the steady-state performance. An experiment is conducted using a commercial vapor chamber sample to confirm the governing mechanisms revealed by the transport model; the model accurately predicts the experimental measurements. Lastly, the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored as a function of the heat spreader thickness and input power. The mechanisms governing the transient behavior of vapor chambers are used to explain the appearance of key performance thresholds beyond which performance is superior to the copper heat spreader. This work provides a foundation for understanding the benefits and limitations of vapor chambers relative to metal heat spreaders in transient operation and may inform the design of vapor chambers for improved transient performance. (C) 2019 Elsevier Ltd. All rights reserved.