Although the “best interest standard” has been widely accepted as a benchmark for decisions made on behalf of others, this chapter explains why medical professionals should eschew the phrase. Decisions by medical professionals are different from decisions by patients and surrogates, and their duty to care for patients and promote their interests is markedly different from acting in the patient’s best interest. Doctors must distinguish their professional fiduciary responsibilities from the concept of best interest because confusing the two can lead to blunders with unfortunate consequences. This chapter responds to supporters of the best interest standard, like Kopelman, and critics, like Veatch, who object to its being subjective and vague. It also explains why introducing the phrase into the language of medical professionals can be misleading or irrelevant and lead to behavior that is intolerant, polarizing, and unjust. Instead, the chapter offers the “three-box model” for guiding doctors’ responses to surrogate decisions.