Aggregation of repeat-containing proteins is associated with neurodegenerative disorders; a specific example is the established link between expansion of the polyglutamine domain in huntingtin and the appearance of nuclear inclusions in Huntington's disease. This connection between aggregation and pathology has motivated numerous investigations into the kinetics of aggregation. Quantitative analysis of kinetic data is needed both for comparative purposes (e.g., to compare the effect of different compounds on aggregation kinetics) and for mechanistic insight. Here we describe some analytical equations that can be used to model aggregation data and demonstrate appropriate and simple methods for extracting valid model parameters by fitting equations to kinetic data.