This document provides an overview of using data.tables in R. It discusses how to create and subset data.tables, manipulate columns by reference, perform grouped operations, and use keys and indexes. Some key points include: - Data.tables allow fast subsetting, updating, and grouping of large data sets using keys and indexes. - Columns can be manipulated by reference using := to efficiently add, update, or remove columns. - Grouped operations like summing are performed efficiently using by to split the data.table into groups. - Keys set on one or more columns allow fast row selection similar to SQL queries on indexed columns.