Travel vaccination requirements depend heavily on your destination, your itinerary within it, and how up to date your routine vaccinations already are. The most common mistake is starting the process too close to departure.
Routine vs. travel-specific
Before thinking about destination-specific vaccines, it's worth confirming your routine vaccinations — tetanus, measles, and similar — are current, since gaps here are surprisingly common in adults. From there, travel-specific vaccines depend on the region: certain areas require or strongly recommend protection against illnesses that aren't a concern at home.
Timing matters
Several travel vaccines need multiple doses spaced weeks apart, and some take up to two weeks after the final dose to become fully effective. Ideally, start checking requirements at least four to six weeks before departure — longer for more remote destinations.
Where to check and keep records
Government travel health websites for your destination are usually the most reliable source for current requirements, since they change based on outbreaks and regional risk. Keep a written or digital record of what you've had and when — it's often required for entry, and useful for any doctor you see while traveling.