Reasons for refusal may be for prior criminality or for medical reasons.
In terms of criminal inadmissibility, something as seemingly minor as an old DUI (Driving Under the Influence) going back a number of years can result in being rejected. Even if the crime was minor in nature, and even if the person’s criminal record has been clean since it took place, Canada still has the right to deny entry.
Some examples of convictions that could make you inadmissible to Canada include: DWAI, Theft, Petty Theft/Larceny, Assault, Drunk & Disorderly Conduct, Obstruction of Justice, Possession of marijuana, cocaine or other controlled substances/drugs, and cautions (issued in the United Kingdom).
More serious reasons for refusal can include membership in terrorist organizations, espionage, participation in war crimes or crimes against humanity, international human rights violations, membership in organized crime groups, criminality, or issues endangering public health, such as tuberculosis.
In 2012–2013, the total number of visa-exempt foreign nationals who arrived in Canada and were deemed inadmissible for entry at air ports of entry was 7,055. This resulted in significant expense, delay and inconvenience for these foreign nationals, other travellers, the airlines and the Canadian government. As such, the eTA system has been brought into operation.