Annual update
Pub 15 (commonly called "Circular E") is reissued each January. It contains the year's wage base for Social Security, Medicare additional wages threshold, federal income tax withholding tables, deposit schedule rules, and the latest electronic-filing thresholds.
Determining employee status
Pub 15 dedicates an early section to the worker classification question: independent contractor vs employee. The factors come from common law (behavioral control, financial control, type of relationship). Misclassification penalties — including back payroll taxes, interest, and FUTA — make this determination one of the highest-stakes decisions an employer makes.
Withholding tables
The publication includes wage-bracket and percentage-method tables for federal income tax withholding on weekly, biweekly, semimonthly, monthly, and other pay periods. Most payroll software applies these tables automatically, but small employers paying by hand should keep a current copy on file.
Deposit schedules
Employers are either monthly or semi-weekly depositors based on the lookback period rule. Pub 15 explains how to determine your status, when deposits are due (15th of next month for monthly; Wednesday or Friday for semi-weekly depending on payday), and the next-day deposit rule for accumulated liabilities of $100,000 or more.
Penalties
Failure-to-deposit penalties range from 2% (1-5 days late) to 15% (after 10 days following IRS demand). Trust fund recovery penalty under Section 6672 makes "responsible persons" personally liable for unpaid withholding tax — including owners, officers, and bookkeepers in some cases.
What the publication does not say
IRS publications are summaries, not the law. They do not cite every controlling regulation, and they routinely omit edge cases that would make the discussion harder to read. For close calls, escalate from the publication to the underlying Internal Revenue Code section, the related Treasury regulations, the relevant revenue rulings or revenue procedures, and (if the dollars warrant) the leading court cases. The IRS-published Internal Revenue Manual and the Audit Techniques Guides — also free on IRS.gov — provide the agency's internal procedural perspective, which often clarifies how the publication's rules play out in an examination.