Do I need a shareholders' agreement if my corporation already has articles of incorporation?

Yes, typically you do, especially if there are multiple shareholders. Articles of incorporation are the public constitutional document and cover the basics, but they cannot anticipate every governance situation. A shareholders' agreement is a private contract among the shareholders that covers matters the articles either cannot or do not address in detail.

A shareholders' agreement typically covers: how decisions are made when shareholders deadlock, restrictions on transferring shares (often mirroring what is in the articles but with more detail), buy-sell provisions if a shareholder wants to exit or dies, drag-along and tag-along rights, non-competition obligations, how key-person disputes are resolved, and the process for valuing shares.

Because the shareholders' agreement is a contract rather than a public corporate document, its terms stay private. It can also override certain default provisions under the Business Corporations Act, which gives you flexibility to tailor governance to your specific situation. For a single-shareholder corporation, a shareholders' agreement is not necessary, but as soon as there are two or more shareholders — especially in a family business or a partnership between unrelated founders — having one in place is strongly recommended. The time to negotiate it is before a dispute arises, not during one.