When a startup accepts money for stock, a SAFE, or a note, it is selling a security, and the federal Securities Act of 1933 applies whether the founder knows it or not. This guide explains the machinery that governs a private raise: the Howey test that decides what counts as a security, the Section 5 registration mandate and the Ralston Purina logic behind every exemption, and the Regulation D safe harbors (Rules 504, 506(b), and 506(c)) that carry the overwhelming majority of American venture dollars. It works through the accredited-investor definition after the 2020 amendments, the public-facing paths in Regulation Crowdfunding and Regulation A+, the general-solicitation line and its pre-existing-relationship escape, the Rule 152 integration framework, restricted securities and Rule 144 resale limits, Form D and blue-sky notice filings, bad-actor disqualification, and the anti-fraud rules that no exemption ever displaces. It closes with the instrument menu, the sources of capital, and a worked seed-round example. Written for founders, counsel, and courts alike, it is general education, not legal advice.