TRACK-HD was a multi-centre, multi-national, prospective, observational study of pre-manifest and early Huntington's disease (HD) with a control group of volunteers not carrying the HD mutation. The goal of the project was to contribute essential methodology that will form the basis for clinical trials in pre-manifest and early HD. TRACK-HD complemented existing observational studies (e.g., Predict-HD, PHAROS, COHORT), sharing some features, but also having areas of unique emphasis.