This paper develops a critical analysis of the constraints that influence the loadability curves of overhead transmission lines (OHLs). The constraints considered are: conductor thermal limit, allowed voltage drop (or voltage quality limit), steady-state (or angle) stability margin, voltage stability margin and Joule losses limit. The analysis points out the different meanings and/or bearing of the various limits. In addition, the paper outlines a general comparison of voltage and angle stability, and discusses the possible role of Joule losses, which are usually not included in line loadability analysis. The paper also discusses how line loadability is affected by the replacement of standard aluminium conductor steel reinforced (ACSR) conductors with high-temperature low-sag (HTLS) conductors, and by reactive compensation with shunt reactors and series capacitors.