System Requirements

From Steam Store page

MINIMUM:
OS: Windows 7 64-bit Processor: Intel Dual-Core 2.4 GHz or AMD Dual-Core Athlon 2.5 GHz

Memory: 4 GB RAM

Graphics: Direct X 9 compliant video card (NVIDIA® GeForce® 9600 GT)

DirectX: Version 9.0c

Network: Broadband Internet connection

Storage: 3 GB available space

Sound Card: DirectX compatible sound card

RECOMMENDED:
OS: Windows 7 64-bit

Processor: Intel Core i5-2300 or AMD Phenom II X4 940 or better

Memory: 8 GB RAM

Graphics: Direct X 11 compliant video card with 2GB of RAM (NVIDIA® GeForce® 560 Ti / ATI Radeon™ 6950 or better)

DirectX: Version 11

Network: Broadband Internet connection

Storage: 3 GB available space

Sound Card: DirectX compatible sound card

MAC OS
MINIMUM:

OS: OS X Lion 10.7

Processor: 2 GHz

Memory: 4 GB RAM

Graphics: equivalent of NVIDIA® GeForce® 9600 GT or better

Network: Broadband Internet connection

Storage: 3 GB available space

RECOMMENDED:

OS: OS X Mavericks

Processor: 3 GHz

Memory: 8 GB RAM

Network: Broadband Internet connection

Storage: 3 GB available space

STEAM OS + LINUX
MINIMUM: OS: Ubuntu 12.04 64-bit

Processor: 2 GHz

Memory: 4 GB RAM

Graphics: Direct X 9 compliant video card (NVIDIA® GeForce® 9600 GT)

Network: Broadband Internet connection

Storage: 3 GB available space

RECOMMANDED:

OS: Ubuntu 14.04 64-bit

Processor: 3 GHz

Memory: 8 GB RAM

Network: Broadband Internet connection

Storage: 3 GB available space