This paper presents a comprehensive summary and detailed analysis on the modeling of modular multilevel converters for High Voltage Direct Current (HVDC) transmission, and develops a state-space switching model for the now well-known Modular Multilevel Converter (MMC), and also for the more recently developed Alternate Arm Converter (AAC). Various models used for the MMC are firstly discussed, followed by the complete derivation procedure of the state-space switching models of both MMC and AAC. The modeling approach used is based on the premise that only state variable transformations and simple voltage and current equations could be formulated, allowing it to capture all circuital information. The only assumption in the derivation is the ideal nature of the switching devices. The paper presents as well an in-depth analysis of the models developed, comparing the MMC and AAC at their most fundamental level of operation.