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It depends on where you want to put the cameras, and why. You must have a reasonable basis for monitoring in this manner (to discourage theft from a cash register and enhance the security of customers, for example) and inform your employees of the cameras. Certain areas of the workplace (the bathroom or changing areas, for example) are generally off-limits to this type of monitoring.
For more information, see Nolo's article Workplace Cameras and Surveillance: Rules for Employers.
For a complete guide to your legal rights and responsibilities as an employer, get The Employer's Legal Handbook: Manage Your Employees & Workplace Effectively, by Fred Steingold (Nolo), which shows you how to comply with the most recent workplace laws and regulations, run a safe and fair workplace, and avoid lawsuits.