Use a semicolon when you want to connect two complete sentences that are closely related without using a coordinating conjunction like and or but. You can also use a semicolon before words like however, therefore, or moreover when those words connect two independent clauses. Semicolons are also useful in lists when the list items already contain commas.

✅ The report is finished; the presentation still needs work.
Both sides could stand alone as complete sentences, which is exactly the kind of structure semicolons are used to connect.

✅ The deadline was tight; however, the team finished on time.
Purdue OWL notes that semicolons are used before conjunctive adverbs such as however, therefore, and moreover when they join two independent clauses.

✅ The offices are in Bogotá, Colombia; Lima, Peru; and Madrid, Spain.
Merriam-Webster notes that semicolons can replace commas in a series when the items themselves already contain commas.

❌ Because the meeting ran late; we missed the train.
A semicolon should not separate a dependent clause from an independent clause. It works best when both sides are complete thoughts. That rule follows from the standard guidance that semicolons connect independent clauses.

The most common mistake is using a semicolon where one side is not a complete sentence. Another common mistake is using a semicolon as if it were just a fancier comma. A semicolon is stronger than a comma and should usually connect two full, closely related thoughts or organize a complex list.

Try this test:

If the answer is yes to both, a semicolon may work.
If the answer is no to one side, you probably need a comma, a conjunction, or a different sentence structure. A second quick memory trick: a semicolon is often “stronger than a comma, weaker than a period.”