Published in the International Journal of Electrical Power and Energy Systems in May, 2020 here. Close coordination between generation and transmission operations and planning is critical to cost effective and reliable energy production and delivery; such coordination, in the presence of ownership diversity, is indeed a primary and challenging goal of regional transmission organizations in the US and similar organizations worldwide. Optimizing these sectors separately overlooks potential synergies that may allow for more effective design and operation of power systems. Coordinated expansion planning (CEP), where both generation and transmission decisions are coordinated, has become especially relevant to present day planning and operations. There are various reasons for this, some of which include the desire to obtain the most environmental and economic benefit from deeper penetration of renewable energy sources, the need for effective deployment of emerging storage technologies, opportunities to capture and harness the electrification of the transport sector, increased interdependencies with other sectors (e.g., gas), and accommodating increased shares of distributed energy resources in distribution grids. These changes result in increased short-term and long-term uncertainties, as well as an increased need for improved representation of multiscale temporal and spatial dynamics (e.g., representing hourly or sub-hourly intertemporal couplings in multi-decadal expansion models). The purpose of this work is to characterize the state-of-the-art in CEP models and identify technical challenges of grid development planning and research and development (R&D) needs for the new generation of these CEP models.