Monolithic 3D (Mono3D) is a three-dimensional integration technology that can overcome some of the fundamental limitations faced by traditional, two-dimensional scaling. This paper analyzes the unique thermal characteristics of Mono3D ICs by simulating a two-tier flip-chip Mono3D IC and highlights the primary differences in comparison to a similarly-sized flip-chip TSV-based 3D IC. Specifically, we perform architectural-level thermal simulations for both technologies and demonstrate that vertical thermal coupling is stronger in Mono3D ICs, leading to lower upper tier temperatures. We also investigate the significance of lateral versus vertical flow of heat in Mono3D ICs. We simulate different hot spot scenarios in a two-tier Mono3D IC and show that although the lateral heat flow is limited as compared to TSV-based 3D ICs, ignoring this mechanism can cause nonnegligible error (∼ 4 ◦C) in temperature estimation, particularly for layers farther from the heat sink. In addition, we show that with increasing interconnect utilization (due to the contribution of Joule heating to overall temperature), the on-chip temperatures and the significance of lateral heat flow within the two-tier Mono3D IC also increase. Finally, we discuss potential opportunities in Mono3D ICs to enhance their thermal integrity.