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In conventional tolerancing, the efforts of designers are mainly directed at selecting suitable values for linear tolerances on part dimensions. These are either determined by trial and error through analysis calculations or optimized according to cost functions. In the transition to geometric dimensioning and tolerancing, the assignment of tolerance values must be preceded by a careful specification of the types of tolerances to be applied on part features. Along with the interrelations among features provided by datum systems, these define a tolerance model which captures design intent and is essential for the allocation and analysis of tolerance values. This chapter reviews the methods available for the specification of geometric tolerances, from common engineering practice to the development of computer-aided support tools. In the description of input data for tolerance specification, special attention is given to design requirements related to fit and function. The general strategy for the resolution of the problem is discussed, with focus on empirical specification rules and tolerance representation models which allow finite sets of tolerancing cases to be classified. The main approaches proposed in the literature for the generative specification of geometric tolerances are described and compared. [ABSTRACT FROM AUTHOR]