A measure of improvement in well-being aggregates increments in the attainment levels of different quality-of-life attributes. This paper first characterizes the entire family of additive improvement indices, where additivity requires that the overall index can be expressed as the arithmetic average of attribute-wise indices. Then we suggest a general family of nonadditive improvement indices, of which the Tsui (1996) index becomes a particular case. Both additive and nonadditive measures are shown to have their respective purposes.