iftop

brew install iftop

For some reason, iftop does not get symlinked to the usual homebrew bin directory, so either symlink it yourself, or move to installation directory.

iftop -i en0

This will start monitoring en0 and display a nice graph with network usage.

tcpdump

If you want to see more traffic information for a specific host, use tcpdump.

sudo tcpdump -i en0 "192.168.1.101"

Alternatively, use wireshark.

MacOS tcpdump

Apple has modified tcpdump to give it the ability to display process information alongside the packets.

sudo tcpdump -i en0 -k N

Here is the list of -k modifiers you can use.

I     interface name (or interface ID)
N     process name
P     process ID
S     service class
D     direction
C     comment
C     flags
U     process UUID (not shown by default)
A     display all types of metadata

nettop

To see a list of processes and their connections, use nettop.

nettop

Press h for the help menu.