Microfit is an interactive econometric software package for microcomputers, and is specifically designed for econometric modelling of time series data. This manual describes what Microfit is, how it works, and what it can do. It makes recent developments in econometric models accessible to students, researchers, and commercial users with its clear presentation of the methods and algorithms underlying the program's features. In addition to a discussion of econometric methods, the manual also contains tutorial lessons, intended to complement the more traditional econometric texts used in quantitative economic courses.