The regional transmission planning and interregional coordination requirements established by FERC through Order Nos. 890 and 1000 represent important tools that regions can wield to address regional transmission needs. Order No. 890 outlined general requirements for local as well as regional transmission planning practices and procedures. Order No. 1000 laid out specific requirements for (1) regional transmission planning; (2) consideration of transmission needs driven by public policy requirements; (3) non-incumbent transmission developer reforms; (4) interregional transmission coordination and cost allocation; and (5) cost allocation for transmission projects that have been selected in a regional transmission plan for purposes of cost allocation. This report is the third in a series of reports on the regional transmission planning practices pursuant to FERC Order Nos. 890 and 1000. The first two reports reviewed regional transmission planning practices and the selection of transmission projects within regions. This report describes interregional transmission coordination practices and activities among one or more regions, focusing on practices for selecting interregional transmission projects for purposes of interregional cost allocation.With respect to interregional transmission coordination, Order No. 1000 directs that public-utility transmission providers in each pair of neighboring transmission planning regions within the Eastern and Western interconnections establish processes for identifying and jointly evaluating interregional transmission projects that may be more efficient or cost-effective solutions to regional needs. Order No. 1000 also requires that each public-utility transmission provider have a method or set of methods for allocating the costs of interregional transmission projects that are selected in both of the relevant regional transmission plans for purposes of cost allocation. The methods must be consistent with six principles FERC outlines for interregional cost allocation. Order No. 1000 does not preclude nor require neighboring regions to produce an interregional plan or to engage in interconnection-wide planning.In response to Order No. 1000, the 12 transmission planning regions recognized by FERC have established compliant transmission coordination processes with one or more of their neighboring regions within the same interconnection. The majority of these processes involve pairs of neighboring regions. One process involves three neighboring regions: ISO-NE, NYISO, and PJM. Another involves all four regions that make up the Western Interconnection: CAISO, ColumbiaGrid, NTTG, and WestConnect. The two earlier reports are Eto (2016): https://emp.lbl.gov/publications/building-electric-transmission-0, and Eto and Gallo (2017): https://emp.lbl.gov/publications/regional-transmission-planning-review.