Both the U.S. and Canada allow their citizens to hold dual citizenship with other countries, and therefore it is legal for you to hold a U.S. and Canadian citizenship while residing in either country. To qualify for Canadian citizenship as a U.S. citizen, you must either be the child of a Canadian citizen, a grandchild of a Canadian citizen, or a spouse of a Canadian citizen. The application process and requirements vary for all cases and connections.
Claim your connection to Canada. If you are a United States citizen and you wish to maintain your citizenship while becoming a Canadian citizen, you will have to either be the child, grandchild, or resident spouse of a Canadian citizen. If you qualify for one of these, you can apply for dual citizenship.
Choose your citizenship application. You can apply for your Canadian citizenship either in Canada or the U.S. The application varies for each situation. If you are an American born to a Canadian born outside of Canada, proceed to step 4, but if you are a spouse of a Canadian, proceed to step 5.
Apply for first generation citizenship identification if you are the child of a Canadian and you were born in the U.S., you must fill out the application (refer to "Citizenship Application" in resources listed below) and send it to the Canadian Consulate General that holds jurisdiction over your area of residence along with your birth certificate, proof of citizenship of your Canadian parent (such as a birth certificate or copy of a current green card), copies of two different forms of identification (either a drivers license or a passport), and any marriage or name change certificates if applicable. Following completion of the application, have taken and include two approved identification photos (refer to "Photograph Specifications" in resources below) and pay the $75 application fee either online at the CIC (Citizenship and Immigration of Canada) or by check or money order.
Apply for second generation Canadian citizenship. This application is for those who have parent(s) who are Canadian citizens that were not born in Canada. The application process is identical to the steps for first generation citizenship, but the applicant must also apply to retain citizenship by either residing in Canada for at least one year prior to the application, or claim a "substantial connection" of either having lived, worked, studied, or performed military service before the grant for citizenship can be made. If a second generation Canadian born outside of Canada fails to acquire citizenship before the age of 28, he will no longer be able to apply for it.
Obtain permanent residency if you are applying for citizenship as a spouse of a Canadian. Marriage to a Canadian does not automatically you for Canadian citizenship. You must apply first for permanent residency (that is a green card) and follow with the application process as outlined in step 3 above.