Reviewing is essential to maintaing your system and building your confidence that you everything has its place and you can find it when you need it.  If you don't do this step - all your work in the first three steps will be lost.  David Allen suggests you do this at a regular time on a weekly basis - either just as you are finishing your week or just as you begin it.  Leo Babauta suggests you also do a quick daily review.  Both are outlined here:

Weekly Review Process:

• Reconnect to your vision as well as your long and short term goals.  Assess how you are doing at forwarding these.
• Develop 2 to 3 goals for the next week. Ask yourself - what is most important to move forward now that will provide the most leverage to move my longer term goals forward.  Make sure you put these goals in a place you will see them and better yet put these ahead of all other items so you focus on them first.
• Update and Tighten Your System
  • Clear out all of your Inboxes (E-mail, Physical, Voicemail, etc.) and straigthen up your work area (collecting)
  • Review the previous week's calendar as well as next week's calendar to see if there are things you have left out and if you find them add them to your lists (e.g. Projects, Next Actions, Waiting For, etc.).
  • Look at any new notes in your collection system (your notebook or PDA) and make sure all items have been moved into your overall system
  • Review your projects and make sure you know your next actions for each project.
  • Consider the next actions you most want to tackle the next week.

Daily Review Process (This should not take more than 15 minutes).  At the end of each day:

• Revisit what you set out to do and assess what you accomplished
• If something fell through the cracks be honest what kept you from it (e.g. distractions, interuptions, etc.)
• Process your Inboxes - keep them clear
• Revisit anything new that came up that day and make sure it gets into your system
• Refocus on your weekly goals
• Refine your plan for the next day