SHIELD Field Training Unit T-Shirt Sheer Go out in the field as an agent of S.H.I.E.L.D. wearing this cool S.H.I.E.L.D. t shirt! The SHIELD Field Training Unit T-Shirt Sheer is heathered gray with a distressed print, a soft feel and a trim fit. This Jack of All Trades t shirt features the logo for the Marvel Comics alien-fighting organization from the series The Avengers, an eagle within a circle, and the words "Field Training Unit."