Power system transmission is at the core of modern wholesale electricity markets, allowing for competition to take place among generators. Thus, its adequate development is of utmost importance to ensure market efficiency with benefits to consumers. However, with the rapid incorporation of massive renewables into current power markets, new economic and technical challenges are arising in transmission planning, building, and integration, with a network needing to change with a faster dynamic faster than the one we were used to. The main challenge is the need for new regulatory procedures and mathematical models to explicitly factor in new uncertainties and risks. Discussions arise on proactive planning, anticipating bulkier transmission paths to cope with risks, identifying adequate cost-allocation schemes, incorporating competitive transmission expansion, and facing socio-environmental challenges. The issue of distributed generation coordinating with transmission expansion appears as a new challenge.