The R package *zoo* provides an S3 class and methods for indexed totally ordered observations, such as both regular and irregular time series. Its key design goals are independence of a particular index class and compatibility with standard generics similar to the behavior of the corresponding "ts" methods. This paper describes how these goals are achieved within *zoo* and provides several illustrations of the available methods for "zoo" objects, including plotting, merging and binding, mathematical operations, data and index extraction and replacement, coercion, and NA handling. A subclass "zooREG" embeds regular time series into the "zoo" framework, bridging the gap between regular and irregular time series classes in R. The package can be used directly by users or as a basic ingredient for more specialized applications.The R package *zoo* provides an S3 class and methods for indexed totally ordered observations, such as both regular and irregular time series. Its key design goals are independence of a particular index class and compatibility with standard generics similar to the behavior of the corresponding "ts" methods. This paper describes how these goals are achieved within *zoo* and provides several illustrations of the available methods for "zoo" objects, including plotting, merging and binding, mathematical operations, data and index extraction and replacement, coercion, and NA handling. A subclass "zooREG" embeds regular time series into the "zoo" framework, bridging the gap between regular and irregular time series classes in R. The package can be used directly by users or as a basic ingredient for more specialized applications.