Get the use of these words wrong, and an editor likely will have to follow up with the admonition to better proofread your writing next time.
Follow-up with a hyphen can be used in two ways:
• An adjective meaning “following an earlier event”: The reporter made a follow-up call to double check his facts.
• A noun meaning “an event that follows an earlier one”: After breaking the news story, the reporter wrote a follow-up for the next issue.
Follow up with no hyphen has a third meaning. It’s a verb showing what one is doing: After covering the fatal accident, the reporter will follow up with a story about the deceased’s life.
“Followup” with no space or hyphen sometimes is used as a replacement for “follow-up.” Editors and instructors generally eschew this spelling, however.
My name is Rob Bignell. I’m an affordable, professional editor who runs Inventing Reality Editing Service, which meets the manuscript needs of writers both new and published. I also offer a variety of self-publishing services. During the past decade, I’ve helped more than 300 novelists and nonfiction authors obtain their publishing dreams at reasonable prices. I’m also the author of the 7 Minutes a Day… writing guidebooks, four nonfiction hiking guidebook series, and the literary novel Windmill. Several of my short stories in the literary and science fiction genres also have been published.