The HART command is sent in a single byte after the expansion bytes in the frame. Commands from a host to a field device can read information from a device such as variables, status, or device information. Commands also can write to a field device to set polling addresses, and write descriptors and messages.

Commands are classified into several categories. First, there are universal commands that are required for all field devices and must be supported for the HART protocol. Second, common-practice commands are implemented for many, but not all devices in a standard format. There are also device-specific commands that can be used for a particular device or device manufacturer. Finally, there are device family commands that can be used for standard functions for a set of device types or manufacturer defined measurements for generic access to the family.

Universal commands are defined for commands 0 - 30, 38, and 48 and describe basic transactions between the host and the field device. Universal commands read device identification, dynamic variables, and other information. These commands also write polling addresses and other identifying information to the device memory. To be compliant with the HART protocol, universal commands must be implemented. Note that with a single byte, basic commands are limited from 0 to 255. Command 31 indicates a 16-bit command number shown in the data field which significantly expands possible commands to a field device.

Common-practice commands are defined from 32 to 121 (except 38 and 48) and are not used in all field devices. When used, common-practice commands must be implemented as described in the protocol. Additional common-practice commands include commands 512 - 767 through expansion.

Commands 122 - 126 are intended for factory use, and 129 and 254 to 511 are reserved. There are additional device-specific commands depending on sensor or actuator type for field devices, and other commands for wireless devices, and WirelessHART within the 16-bit expansion.