Robert "Bobby" Costello started as the U.S. Eventing Team Chef d'Equipe and Technical Advisor for the program in 2022. With a decorated career as an international athlete, actively competing up until 2010, Costello was a member of the World Championship Team and placed in the top 10 as an individual at the Sydney Olympic Games in 2000. As the U.S. Technical Advisor, Costello led the U.S. Eventing Team to several key championship results, including a team silver at the 2022 FEI World Eventing Championships in Pratoni del Vivaro Italy and a team silver at the 2023 Pan American Games in Santiago, Chile.  

James Alliston is a dedicated eventing athlete and trainer, known for his commitment to developing both horses and riders. He grew up in Gloucestershire, England, where he was involved in Pony Club, honing his skills in various disciplines. After taking a gap year from university to immerse himself in the horse industry, Alliston apprenticed for U.S. Eventing athlete Bruce Davidson, Sr. This experience influenced his training philosophy of emphasizing the importance of producing fit, competitive horses. 
 
In early 2022, Alliston changed his nationality from British to American after residing in the U.S. for over 15 years. Soon after, he was selected to represent the U.S. Eventing Team at the FEI Eventing Nations Cup in Boekelo, Netherlands. He and Alliston Equestrian’s 8-year-old gelding, Nemesis, served as pathfinders for the team. 
 

Over the years Alliston has continued to achieve success at the CCI4* and CCI5* levels. In 2023, he once more represented the U.S. Eventing Team at Boekelo, this time riding Ric Plummer and Alliston Equestrian’s mare, Karma, where they finished on their dressage score of 35.9, leading the U.S. Eventing Team to a fourth-place finish. Alliston and Karma were named to the Defender U.S. Eventing Team for CHIO Aachen in 2024. The pair not only produced a double clear show jumping round, but assumed the role as pathfinders and crossed through the timers with just 2.4 time penalties to finalize their outing on a score of 38.4, finishing ninth overall individually and as the highest-placed team combination in the 2024 SAP-Cup CCIO4*-S. 
 
James owns and operates Alliston Equestrian and East Bay Riding Academy in Castro Valley, Calif., where he trains alongside his wife Helen. 

California-born Elisabeth “Liz” Halliday began riding when she was 8 years as an active member the United States Pony Club. Halliday moved to England in 2000 to work for famed eventer William Fox-Pitt and was based in Europe for many years before bringing her operation to the U.S. with locations in Ocala, Florida, and Lexington, Kentucky, in 2020. That same year, she earned the distinction of USEA Rider of the Year after racking up nine wins and 23 total top-ten finishes at FEI competitions over the season.

Halliday achieved her first CCI5*-L completion in 2016 at the Kentucky Three-Day Event with Deborah Halliday's HHS Cooley. She has also brought numerous horses to top-ten finishes at the level, including Ocala Horse Properties, LLC's Deniro Z, The Monster Partnership's Cooley Quicksilver, and The Nutcracker Syndicate's Cooley Nutrcracker.

In 2023, she brought Deborah Palmer's and Ocala Horse Properties, LLC's Miks Master C to his CCI5*-L debut at the Kentucky Three-Day Event where they finished in third-place overall and were reserve champions in the USEF CCI5*-L Eventing National Championship. In 2023, the pair were the Adequan USEA Advanced National Champions at the American Eventing Championships.

Halliday has represented the U.S. Eventing Team in several FEI Nations Cup competitions, including the 2022 CCIO4*-S at Bromont in Canada, where she rode Miks Master C to individual victory and led the U.S. team to secure the win. In 2023, the pair were part of the silver medal-winning team at the Pan American Games in Santiago, Chile.

Halliday continued her consistency in 2024, marking several top results with both Cooley Nutcracker and Miks Master C, with a top-ten finish coming for Cooley Nutcracker at the Kentucky Three-Day Event CCI5*-L in April. The pair were called onto the U.S. Olympic Eventing Team for the Paris 2024 Olympic Games where they helped the team finish sixth overall and 19th individually. 

In addition to her equestrian pursuits, Halliday was a professional driver in sportscar and GT endurance disciplines, competing from 1997 to 2010 before focusing on her equestrian career. 

Boyd Martin, the son of Olympic speed skater Toy (Dorgan) Martin and Olympic cross-country skier Ross Martin, began riding at a young age in his native country of Australia. He made his debut at the CCI5*-L with a win at the 2003 Australian International Three-Day Event riding the Thoroughbred gelding True Blue Toozac.

He moved to the U.S. in 2007 to further his career as an eventing professional, serving as the assistant trainer to Olympian Phillip Dutton for two years before starting his own business. Martin began riding for the U.S. in 2009.

Martin is a three-time Olympian, having represented the U.S. in London 2012, Rio de Janeiro 2016, and Tokyo 2020. He is a four-time U.S. Eventing Team member for the FEI Eventing World Championship, including as a part of the silver medal-winning team in 2022 in Pratoni, Italy. With Pancho Villa, he was a member of the U.S. team that won gold at the 2015 Pan American Games, and in 2019, he won team and individual gold at the Pan American Games with Christine, Thomas, and Tommie Turner's Tsetserleg TSF.

Twice, Martin has been the USEF CCI5*-L Eventing National Champion at the Kentucky Three-Day Event. First in 2019 with Tsetserleg TSF and again in 2021 with Christine, Thomas, Tommie, and his own, On Cue. In October 2021, he won the inaugural Maryland 5 Star with On Cue, becoming the first American to win a CCI5* since 2008.

In 2023, Martin piloted his string to several significant top finishes at the CCI5*-L with five top-ten finishes at events around the world, including two top-ten finishes for Fedarman B at both Luhmuhlen and Pau. Martin brought both Fedarman B and Commando 3 forward as potential Olympic selections at the start of the 2024 season. A strong showing of both at the CCI4*-S at the 2024 Kentucky Three-Day Event did not make selection easy, but Martin ultimately chose Fedarman B as his mount of the 2024 Olympic Games, with Commando 3 as a direct reserve. Paris 2024 will be Martin's fourth consecutive Olympic appearance for the U.S. Olympic Eventing Team. 

Martin and Fedarman B were successful in Paris, adding just 1.6 to their initial dressage score of 30.5. The duo were one of only seven combinations to jump clear in both rounds on the final day, finishing tenth individually and contributing to the team's sixth place. 

In 2025, Martin helped the Defender U.S. Eventing team secure a second-place finish for the third consecutive year at the CHIO Aachen CCI4*-S and later that fall, earned the first ever title as winner of the Inaugural US Equestrian Open of Eventing Final held at Morven Park. Martin also placed on the podium at every CCI5*-L entered in 2025 and subsequently was named the 2025 World Equestrian Brands USEA Rider of the Year. 

The 2026 season started strong for Martin with a podium finish aboard Commando 3, owned by Yankee Creek Ranch, at the Bouckaert CCI4*-S. Martin then claimed a second and third-place finish on Miks Master C, owned by OHP Hammertime Partners, and Shanroe Cooley, owned by Dallas LLC, at the Fair Hill International CCI4*-S the following month. In April 2026, Martin had two strong finishes with Commando 3 and Cooley Nutcracker at the Defender Kentucky Three Day Event CCI5*-L, earning sixth place with Commando 3, and finishing just outside of the top ten with Cooley Nutcracker, owned by Liz’s Nutcrackers.  The following month, Martin and Shanroe Cooley took home third in the Tryon International CCI4*-S before making their first 5* appearance at the Longines Luhmühlen Horse Trials in June coming home with yet another podium finish, earning third place overall.  

Martin and his wife, professional dressage trainer and athlete, Silva Martin, own and operate Windurra USA in Pennsylvania.

Phillips began her equestrian pursuits at a young age, initially riding Western before transitioning to English riding and eventually eventing at around six years old. Drawn to the thrill of cross-country and jumping, Alyssa pursued her passion for horses throughout her childhood and into college. 
 
Alyssa Phillips had a successful young rider career, winning individual gold at the 2011 Adequan®/FEI North American Youth Championships presented by Gotham North, team gold at the NAYC in 2011 and 2012, and team silver in 2014. With Bliss III, Phillips had top-10 finishes in CCI4*-S events at Rebecca Farm in 2015 and Richland Park in 2016. In 2017, Alyssa began training with Jennie Brannigan, gaining valuable experience in the competitive world of eventing. With her Holsteiner gelding, Oskar, she won the CCI3*-L U25 at Bromont in 2019 and had top ten finishes in CCI3*-S events at Carolina International and Plantation Fields and in the CCI4*-S at Ocala Jockey Club. Phillips has been a member of the USEF Eventing High Performance Programs for several years and has represented the Development Team in international competition. 

Phillips has established her own operation in Ocala, Florida, where she manages her horses and staff, finding the experience both challenging and rewarding. 
 
Phillips was selected as the traveling reserve for the Pan American Games in 2023, an opportunity that provided significant learning experiences despite facing setbacks that year. She participated in the development tour, achieving noteworthy results, including a second-place finish at Millstreet. Career highlights include winning the 2021 Land Rover Kentucky Three Day Event CCI4*-S, winning the 2023 Rebecca Farm CCI3*-S, and winning the 2024 Event at TerraNova CCI4*-S with her horse Oskar.