Apache Cassandra is a wide-column, open source NoSQL database and the most commonly used NoSQL database in its category. The container of data, equivalent to a database schema in a relational database, in Apache Cassandra is a Keyspace. The basic unit of storage is a column family (also called table), and each record in a table is stored in a row with the data being stored in columns. A column has a name, a value, and a timestamp associated with it. A column is not required to store a value and the column could be empty. Apache Cassandra is based on a flexible schema (or schema-free or dynamic schema) data model in which different rows could have different columns and the columns are not required to be pre-specified in a table definition. Apache Cassandra supports data types for column names (called comparators) and column values (called validators), but does not require the data types (validators and comparators) to be specified. The validators and comparators may be added or modified after a table (column family) has been defined. Apache Cassandra provides a Cassandra Query Language (CQL) for CRUD (add, get, update, delete) operations on a table. Apache Cassandra installation includes a cqlsh utility, which is an interactive shell, from which CQL commands may be run. An official Docker image for Apache Cassandra is available and in this chapter we shall run Apache Cassandra in a Docker container.