Plato's works, perhaps the most consistently popular and influential philosophic writings ever published, consist of a series of dialogues in which the discussions between Socrates and others are presented with infinite charm. Most of our knowledge of Socrates is from these dialogues, and which views are Socrates' and which are Plato's is anybody's guess.
Plato cautiously never introduced himself into any of the dialogues. The works that have been transmitted to us through the middle ages under the name of Plato consist in a set of 41 so-called "dialogues" plus a collection of 13 letters and a book of Definitions. But it was already obvious in antiquity that not all of these were from Plato's own hand.
The exact ordering of the dialogues is not known, but they can be roughly assigned to three periods, the early, middle, and late. The early dialogues, began after 399 B.C., are seen by many as memorials to the life and teaching of Socrates.
Euthyphro, Apology, Crito, Phædo
Cratylus, Theætetus, Sophist, Statesman
Parmenides, Philebus, Symposium, Phædrus
Alcibiades, 2nd Alcibiades, Hipparchus, Rival Lovers
Theages, Charmides, Laches, Lysis
Euthydemus, Protagoras, Gorgias, Meno
Hippias major, Hippias minor, Ion, Menexenus
Clitophon, Republic, Timæus, Critias
Minos, Laws, Epinomis, Letters
But the same Diogenes mentions also a grouping in trilogies (groups of three), which he attributes to Aristophanes of Byzantium (3rd century BC) and which covers only a subset of the dialogues. This one goes as follows :
Republic, Timæus, Critias
Sophist, Statesman, Cratylus
Laws, Minos, Epinomis
Theætetus, Euthyphro, Apology
Crito, Phædo, Letters
The tetralogies of Greek theater were made up of one comedy and a trilogy of tragedies. If there is anything in the idea that Plato grouped his dialogues according to such an arrangement, it might explain why we sometimes hear of tetralogies, sometimes of trilogies.
Plato, The Collected Dialogues including the Letters, edited by Edith Hamilton and Huntington Cairns, with Introduction and Prefatory Notes, Bollingen Series LXXI, Princeton University Press, 1961.
Apology, translated by Hugh Tredennick,
Charmides, translated by B. Jowett,
Cratylus, translated by B. Jowett,
Critias, translated by A. E. Taylor,
Crito, translated by Hugh Tredennick,
Epinomis, translated by A. E. Taylor,
Euthydemus, translated by W. H. D. Rouse,
Euthyphro, translated by Lane Cooper,
Gorgias, translated by W. D. Woodhead,
Hippias Major (or Greater Hippias), translated by B. Jowett,
Hippias Minor (or Lesser Hippias), translated by B. Jowett,
Ion, translated by Lane Cooper,
Laches, translated by B. Jowett,
Laws, translated by A. E. Taylor,
Letters, translated by L. A. Post,
Lysis, translated by J. Wright,
Menexenus, translated by B. Jowett,
Meno, translated by W. K. C. Guthrie,
Parmenides, translated by F. M. Cornford,
Phædo, translated by Hugh Tredennick,
Phædrus, translated by R. Hackforth,
Philebus, translated by R. Hackforth,
Protagoras, translated by W. K. C. Guthrie,
Republic, translated by Paul Shorey,
Sophist, translated by F. M. Cornford,
Statesman, translated by J. B. Skemp,
Symposium, translated by Michael Joyce,
Theætetus, translated by F. M. Cornford,
Timæus, translated by B. Jowett,
Plato, Complete Works, Edited, with Introduction and Notes, by John M. Cooper, Hackett Publishing Company, Indianapolis/Cambridge, 1997.
Alcibiades, translated by D. S. Hutchinson
Alcibiades (2), translated by Anthony Kenny
Apology, translated by G. M. A. Grube
Axiochus, translated by Jackson P. Hershbell
Charmides, translated by Rosamond Kent Sprague
Clitophon, translated by Francisco J. Gonzalez
Cratylus, translated by C. D. C. Reeve
Critias, translated by Diskin Clay
Crito, translated by G. M. A. Grube
Definitions, translated by D. S. Hutchinson
Demodocus, translated by Jonathan Barnes
Epigrams, translated by J. M. Edmonds, rev. John M. Cooper
Epinomis, translated by Richard D. McKirahan, Jr.
Eryxias, translated by Mark Joyal
Euthydemus, translated by Rosamond Kent Sprague
Euthyphro, translated by G. M. A. Grube
Gorgias, translated by Donald J. Zeyl
Halcyon, translated by Brad Inwood
Hipparchus, translated by Nicholas D. Smith
Hippias Major (or Greater Hippias), translated by Paul Woodruff
Hippias Minor (or Lesser Hippias), translated by Nicholas D. Smith
Ion, translated by Paul Woodruff
On Justice, translated by Andrew S. Becker
Laches, translated by Rosamond Kent Sprague
Laws, translated by Trevor J. Saunders
Letters, translated by Glenn R. Morrow
Lysis, translated by Stanley Lombardo
Menexenus, translated by Paul Ryan
Meno, translated by G. M. A. Grube
Minos, translated by Malcolm Schofield
Parmenides, translated by Mary Louise Gill and Paul Ryan
Phaedo, translated by G. M. A. Grube
Phaedrus, translated by Alexander Nehamas and Paul Woodruff
Philebus, translated by Dorothea Frede
Protagoras, translated by Stanley Lombardo and Karen Bell
Republic, translated by G. M. A. Grube, rev. C. D. C. Reeve
Rival Lovers, translated by Jeffrey Mitscherling
Sisyphus, translated by David Gallop
Sophist, translated by Nicholas P. White
Statesman, translated by C. J. Rowe
Symposium, translated by Alexander Nehamas and Paul Woodruff
Theaetetus, translated by M. J. Levett, rev. Myles F. Burnyeat
Theages, translated by Nicholas D. Smith
Timaeus, translated by Donald J. Zeyl
On Virtue, translated by Mark Reuter