We discuss methods for modelling multivariate autoregressive time series in terms of a smaller number of index series which
are chosen to provide as complete a summary as possible of the past information contained in the original series necessary
for prediction purposes. The maximum likelihood method of estimation and asymptotic properties of estimators of the coefficients
which determine the index variables, a well as the corresponding autoregressive coefficients, are discussed. A numerical example
is presented to illustrate the use of the autoregressive index models.