This is a complex topic, and one that will have debatable answers depending on your personal position. Legally, covert intelligence collection or surveillance is permissible if proper legal channels are adhered to. For example, anti-terrorism laws provide greater latitude to gather phone data, bank records, etc. On the other hand, an individual entity simply deciding to conduct a warrantless wiretap would be illegal.
If you would like to discuss this in greater detail, feel free to contact me to schedule a lesson.