Commas separate the elements in series of three or more:

Ransom notes, spoken or written threats, emergency calls, online scams, and police interrogations all provide forensic evidence.

When writing a series, make sure that all of its elements are parallel.

Some writers (particularly journalists) don’t use the serial comma — the one that comes before and (or or) and the final element of a series. You don’t need to if you find it more natural to leave it out. Omitting a serial comma, however, can lead to confusion:

If you make a habit of using serial commas, your readers will understand your meaning whenever you write a series.

As with nouns presented in a series, two or more adjectives or adjective phrases require commas between them if each one modifies the noun separately:

Detectives are only beginning to make sophisticated, effective use of language to clear the innocent and catch the guilty.

To test whether adjectives are separate, mentally replace the comma with and or but:

Detectives are only beginning to make sophisticated and effective use of language to clear the innocent and catch the guilty.

If one of these replacements has little or no effect on the meaning, use a comma. If, however, the second adjective is an integral part of the following noun, don’t use a comma:

But scientific forensic linguistics has tremendous untapped potential.

Here it’s forensic linguistics that’s being called scientific, and the untapped potential that’s being called tremendous. (You wouldn’t say scientific and forensic linguistics or tremendous and untapped potential.)