Phase 1, Phase I or Phase One may refer to:

• Marvel Cinematic Universe: Phase One, six American superhero films from 2008–2012
• Phase One (Art Ensemble of Chicago album), 1971
• Phase One (Saga album), 1998
• Phase One, renamed Bloodwork (film), a 2012 American-Canadian thriller

• Phase I clinical trials, the first of the phases of clinical research
• Canadian Phase I, a powered parachute design
• Phase I metabolism, Phase I reaction in drug metabolism
• Phase I environmental site assessment, a report prepared for a real estate holding which identifies potential or existing environmental contamination liabilities
• Phase 1 (bar), a lesbian bar and nightclub at 525 8th Street, Southeast in Washington, D.C
• Phase One (company), a Danish camera equipment company
• Phase 1 metro station
• Phase One Media Pro
• Phase One Endurance

• Phase (disambiguation)