As the result of post 9/11 agreements between the U.S. and Canada, Canadian border officials now have computer access to many U.S. criminal records. As a result, Americans with criminal records are more frequently experiencing problems at the Canadian border.
Even if you have been crossing the border without problems in the past, you can now be denied entry into Canada (even for a visit) if you have a criminal record, no matter how minor or how ancient. Driving under the influence, shoplifting, driving dangerously and possession of illegal substances are all reasons to be considered "criminally inadmissable" to Canada. Traffic tickets do not make you inadmissable. You can be inadmissable for entry into Canada even if you were not convicted but the crime would have been one for which you could have been convicted.
Citizenship and Immigration Canada provides a FAQ site
dealing with criminal inadmissability. There are a number of ways to overcome a past conviction, but it will add considerable time to any immigration process. It is highly recommended that you consult a Canadian immigration lawyer about any criminal offence before trying to enter Canada, allowing for ample lead time. The Faculty Recruitment and Support office can provide you with names of lawyers, confidentially.
Other Reasons for Inadmissibility
Individuals may also be denied a visa or refused entry into Canada for a variety of other reasons, including security reasons, health reasons, misrepresentation or having a family member who is inadmissable. For a complete list of grounds, see the Citizenship and Immigration web site.