The Mental Toughness Questionnaire-48 (MTQ48; Earle, 2006) is the most utilised instrument to measure Mental Toughness (MT) in sport (Gucciardi, Hanton, & Mallett, 2012). To date, preliminary research examining the factorial validity of the MTQ48 has yielded equivocal findings (Gucciardi et al; Horsburgh, Schermer, Veselka, & Vernon, 2009) regarding its ability to measure the 4/6C’s model of MT (Clough, Earle, & Sewell, 2002; Earle). The primary aim of this thesis was to provide a truly comprehensive examination of the factorial validity of the MTQ48 in an effort to provide an adequate measure of the 4/6C’s model of MT. Study 1 re-examined the factor structure of the MTQ48 by using a very large sample of competitive student athletes and found little support for its factorial validity. Inspection of item content revealed concerns regarding the adequacy of MTQ48 items to represent the 4/6C’s model of MT. Study 2 developed the University of Chichester Mental Toughness Questionnaire (UCMTQ) whereby items were generated to better represent the 4/6C’s model of MT. Although the UCMTQ’s factorial validity was superior to the tested models of the MTQ48, the results of Study 2 provided little support for its factorial validity. It was concluded that the poor factorial validity of the UCMTQ could have been due to the inadequacy of the factor definitions developed by Clough, Marchant, and Earle (2007) to represent the core traits underpinning the 4/6C’s model of MT (challenge, commitment, control, and confidence).