Server: Atlassian JIRA, version from 4.2 to 6.0+, including JIRA OnDemand. (Please use JIRA Client 3.0.2 with JIRA 5.1.8 and older versions.)
JIRA Remote API: should be enabled. It is most likely already turned on if your JIRA is used with any remote tool, though it is off by default. Remote API access is not required by JIRA Client for read-only access, but it is required for uploading any changes.
To turn Remote API on, JIRA administrator needs to enable the "Accept remote API calls" setting in the JIRA configuration, General section.
Additional server Authentication: Basic HTTP, Digest, NTLM version 1 or NTLMv2.
Operating System: Microsoft Windows (XP/Vista/7/8), Linux/x86 or AMD64, Apple Mac OS X (Leopard/Snow Leopard/Lion), or any other that supports Java 6.
System Memory: 256 MB required, 512 MB recommended.
Hard Drive Space: 150 MB required, 300 MB recommended.
Screen Resolution: 1024x768 or better.
Displays: multiple displays are supported, but JIRA Client should be restarted when a display is connected or disconnected.
Network Connection: Atlassian JIRA produces a lot of traffic when answering JIRA Client requests, so performance over slow network connection may be degraded. (Hint: turn on "Use Gzip Compression" setting in JIRA configuration to reduce traffic.)
Java: If you downloaded a distribution without bundled Java, you need Java 6 or later available at java.com.