Place a semicolon between two complete sentences (also known as independent clauses) when the second sentence compliments the first, or is a restatement of what was in the first sentence. Example: My dog loves to chase squirrels; she also loves to chase chipmunks.
Separate two sentences with a semicolon when the second sentence starts with a conjunctive adverb (thus, moreover, furthermore, nevertheless, meanwhile, consequently, otherwise, however, therefore, nonetheless). Example: It has been raining everyday this week; therefore, the fields are flooded.
Use a semicolon between two sentences when the second sentence starts with a transition (that is, in addition, on the other hand, for example, even so, in fact, however, in other words). Example: Severe thunderstorms are dangerous; in fact, they can spawn tornadoes without warning.
A semicolon should be used in a sentence where a series of items already include commas. Example: The movie industry has grown in Atlanta, Georgia; New York, New York; Detroit, Michigan; and Chicago, Illinois.
Semicolons are used to separate two sentences that already contain commas. Example: Her cat needed to see the vet for de-clawing, de-worming, shots and blood work; I don't know how she affords all that veterinary care.
Consider how much of a pause you want between sentences if they were to be read aloud. If you want a brief pause then use a comma. For a little longer pause use a semicolon. If there should be a full stop between the sentences then a period is appropriate.
Semicolons are not needed when there is a conjunction (but, nor, so, and, or, for, yet) connecting two sentences. Example: I don't like most melons, but I do enjoy watermelon.