|State Highway 121|
|Length:||85.56 mi (137.69 km)|
|Southwest end:||I-35W/US 287 at Fort Worth|
|Northeast end:||SH 78|
State Highway 121, locally known as 121, is a diagonal state highway, angling from southwest to northeast through north central Texas. It runs from downtown Fort Worth, Texas at the junction of Interstate 35W to Bonham, Texas, just north of a junction with U.S. Highway 82.
Sections between downtown Fort Worth and Grapevine, Texas are freeway, including a small segment near Hurst, Texas that coincides with Interstate 820. At Bedford, Texas it has an interchange with State Highway 183, the Irving freeway that leads toward Dallas; it has an interchange with State Highway 360, which leads toward Arlington, Texas, just to the west of the north entrance to the Dallas/Fort Worth International Airport, to which State Highway 121 gives access. At Grapevine it has interchanges with State Highway 114 and Interstate 635 before becoming a limited-access tollway that passes between Lewisville and Frisco.
It is very heavily traveled south and west of McKinney and has become an urban highway due to the rapid growth of the Dallas-Fort Worth Metroplex. Sections not yet of freeway quality are scheduled for upgrade or replacement as a freeway; between Frisco and McKinney service roads are in place along with one overpass at Custer Road. Service roads extending southwest to northeast along the corridor from Lewisville to McKinney are free of charge, however the main lanes within the right-of-way are tolled as part of the Sam Rayburn Tollway operated by the North Texas Tollway Authority (NTTA.) The highway's northeastern half is primarily two lanes wide and carries significantly less traffic.
It meets Interstate 35E at Lewisville, Texas, State Highway 289 near Frisco, Texas, U.S. Highway 75 and U.S. Highway 380 in McKinney, Texas, U.S. Highway 69 at Trenton, Texas, and State Highway 78 just north of Bonham.
In March, 2009, work was completed on a short stretch of TX 121 northeast of DFW Airport which upgraded 2 signalized at-grade intersections to a freeway, with overpasses at both intersections, clearing up a problem for rush-hour traffic.
The Sam Rayburn Tollway (State Highway 121 Toll or SH 121T) is a toll facility to be built as the main lanes of State Highway 121 in Collin County, Texas. The North Texas Tollway Authority (NTTA), in conjunction with the Texas Department of Transportation (TxDOT), has studied potential options for constructing SH 121 as a tollway between the Dallas North Tollway and US 75 (Central Expressway), a distance of approximately 12 miles (19 km).
Tolls are collected along the entire route by using only electronic toll collection with no tollbooths or toll collector, like the Westpark Tollway in Houston. People will be able to use a transponder unit known as a TollTag attached to a vehicle's windshield, which communicates with overhead sensors to deduct tolls from the user's toll account. If a person does not have a TollTag, they can still drive on the tollway, because the overhead sensors will scan and record the licence plate number, locate the owner's address, and send a monthly bill showing their toll cost plus 1/3 of a toll cost (to cover the license plate processing) plus a $1.00 invoice fee. 
The original portion of the toll road from Business 121 near Coppell to FM 2281 opened in May 2006, and toll collection started December 1 of that year. The tolled portion extended to Hillcrest Rd. in Plano/Frisco on August 31, 2008. The tollway was extended to Custer Rd. on September 1, 2009, and again to Hardin Blvd. in McKinney on October 1, 2009.
In early 2007, the Spanish firm Cintra agreed to a $2.8 billion, 50-year deal to finish building and maintain SH 121. However, critics of the deal have questioned both its length—the contract allows Cintra to collect tolls for 50 years—and the fact that the North Texas Tollway Authority (NTTA) was prevented from bidding on the contract. In addition, general opposition to the toll plan organized, pointing out that the segment through Denton County had already been paid for.
To answer some of these complaints, Texas legislators pressed the Texas Department of Transportation to allow the NTTA to bid. The Authority announced their proposed bid in early May 2007, which would provide approximately $3.3 billion in road funding, but is based on traffic counts that differ from those used by Cintra.
On June 28, 2007, the Texas Transportation Commission announced it would pursue the proposal from the North Texas Tollway Authority (NTTA). 
There are no mileposts on any state freeway in Texas. The exits are shown from southwest to northeast.
|Tarrant||Fort Worth||Spur 347 west (Belknap Street) – Downtown Fort Worth||Continuation beyond I-35W|
|I-35W / US 287 / US 377 south – Denton, Waco|
|US 377 north (Belknap Street)||Northbound exit and southbound entrance|
|Maxine Street||Northbound exit and southbound entrance|
|Richland Hills||Handley-Ederville Road|
|I-820 south||South end of I-820 overlap|
|SH 10 east / SH 183 west – Richland Hills||South end of SH 183 overlap; signed as exit 24 southbound|
|Pipeline Road, Glenview Drive||Signed as exit 23 eastbound|
|North Richland Hills|
|SH 26 / FM 1938 north (Davis Boulevard) – Colleyville, Tarrant County College Northeast Campus||Signed as exit 22A northbound|
|I-820 west||North end of I-820 overlap|
|Bedford-Euless Road||Signed as exit 22A northbound|
|FM 3029 north (Precinct Line Road) / Hurstview Drive|
|Norwood Drive, Brown Trail|
|Bedford||Bedford Road, Forest Ridge Drive, Brown Trail|
|Central Drive, Forest Ridge Drive|
|SH 183 east – D/FW Airport South Entry, Irving||No southbound exit; north end of SH 183 overlap|
|Murphy Drive, Euless Westpark Way||Northbound exit is via SH 183 east|
|FM 157 south / Cheek-Sparger Road|
|Stone Myers Parkway||Northbound exit and southbound entrance|
|SH 360 south – Arlington||Southbound exit and northbound entrance|
|William D. Tate Avenue||Southbound exit is via SH 114 west|
|SH 114 west – Bridgeport||South end of SH 114 overlap; northbound exit is via Tate Avenue|
|Spur 103 (Main Street)|
|SH 114 Bus. west (Texan Trail) to SH 26|
|SH 114 east – Irving, Dallas||North end of SH 114 overlap|
|Bass Pro Drive|
|FM 2499 north (Grapevine Mills Parkway) – Flower Mound||No northbound entrance|
|Sandy Lake Road / Grapevine Mills Parkway / Freeport Parkway / SH 26 south||SH 26 accessible from south only|
|SH 121 Bus. north – Lewisville|
|Denton Tap Road||The tolled stretch of 121 begins after this exit|
|Lewisville||I-35E (US 77) – Denton, Dallas||Northbound exit to I-35E north is via MacArthur Boulevard exit|
|Marchant Boulevard, Huffines Boulevard|
|SH 121 Bus. south / FM 2281 – Lewisville|
|FM 423 (Main St.)||to Little Elm|
|Plano Parkway/Paige Road|
|Plano||Spring Creek Parkway|
|Dallas North Tollway|
|SH 289 (Preston Rd.)|
|Hillcrest Road/Robinson Road|
|Lake Forest Drive/Watters Road|
|Hardin Boulevard||SRT opened to this point October 1, 2009.|
|US 75 (North Central Expressway)||Multiplexes with US75 to Melissa, TX at US75 exit 45.|
|Eldorado Parkway||Signed as exit 39|
|Spur 359 / FM 3038 (Louisiana Street/Virginia Parkway.)||Signed as exit 40A|
|White Avenue||Signed as exit 40B|
|US 380 (University Drive)||Signed as exit 41|
|Wilmeth Road||Signed as exit 42|
|FM 543 (Weston Road)||Signed as exit 43|
|Telephone Road/Davis Road||Signed as exit 44|
|US 75 (North Central Expressway)||Signed as exit 45 (End of US75/TX121 multiplex)|
|SH 5||At-Grade Interchange|
|FM 545 (Sherman Street)||At-Grade Intersection|
|FM 2933||At-Grade Intersection|
|FM 455 (White Street)||At-Grade Intersection|
|FM 2662 (Houston Street)||At-Grade Intersection|
|FM 160||At-Grade Intersection|
|FM 819||At-Grade Intersection|
|US 69||Diamond Interchange|
|SH 121 Bus.||At-Grade Intersection|
|SH 11||Diamond Interchange|
|FM 1629||At-Grade Intersection|
|SH 311||At-Grade Interchange|
|SH 56||At-Grade Interchange|
|US 82||Diamond Interchange|
|FM 898||At-Grade Intersection|
|SH 78||At-Grade Interchange (Northern Terminus)|