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                                                                   EXHIBIT 5.1

                            KELLEY DRYE & WARREN LLP
                                 101 Park Avenue
                            New York, New York 10178

                                                       August 28, 2000

Viatel, Inc.
685 Third Avenue
New York, New York 10017

                  Re:  Registration Statement on Form S-4

Ladies and Gentlemen:

         We have acted as counsel to Viatel, Inc., a Delaware corporation (the
"Company"), in connection with the preparation and filing of a Registration
Statement on Form S-4 (the "Registration Statement") with the Securities and
Exchange Commission (the "Commission") pursuant to the Securities Act of 1933,
as amended (the "Act"), with respect to the proposed offer by the Company to
exchange (the "Exchange Offer") E300,000,000 principal amount of its
12 3/4% Senior Euro Notes Due 2008 (the "Exchange Notes") for a like aggregate
principal amount of its outstanding 12 3/4% Senior Euro Notes Due 2008 (the
"Existing Notes") which have not been registered under the Act. The Exchange
Notes will be issued pursuant to the Indenture, dated as of April 20, 2000,
between the Company and The Bank of New York, as Trustee, pursuant to which the
Existing Notes were originally issued (the "Indenture"). As such counsel, you
have requested our opinion as to the matters described herein relating to the
issuance of the Exchange Notes in the Exchange Offer.

         In connection with this opinion, we have examined and relied upon: (i)
the Company's Amended and Restated Certificate of Incorporation, as amended, as
certified by governmental officials, and the Company's Third Amended and
Restated By-laws, as certified by the Secretary of the Company; (ii) the minute
books and other records of corporate proceedings of the Company, as made
available to us by officers of the Company; (iii) an executed copy of the
Registration Statement, together with the exhibits and schedules thereto, in the
form filed with the Commission; (iv) an executed copy of the Indenture; and (v)
such other records, agreements, certificates, instruments and information from
officers and other representatives of the Company and governmental and
regulatory authorities as we have deemed necessary as the basis for the opinion
expressed herein; and we have reviewed such matters of law deemed necessary by
us in order to deliver the within opinion.

         For purposes of this opinion, we have assumed the authenticity of all
documents submitted to us as originals, the conformity to originals of copies
and the authenticity of the originals of such copies. We have also assumed the
legal capacity of all natural persons, the genuineness of all signatures on all
documents examined by us, the authority of such persons signing on behalf of the
parties thereto other than the Company and the due authorization,


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execution and delivery of all documents by the parties thereto other than the
Company. We express no opinion with respect to the enforceability of any
provision of the Indenture or the Exchange Notes to the extent such provision
may be subject to, or affected by, (i) applicable bankruptcy, insolvency,
reorganization, moratorium or similar state or federal law affecting the rights
and remedies of creditors generally (including, without limitation, fraudulent
conveyance laws) or (ii) equitable principles (including, without limitation,
the duty to act in good faith). We express no opinion with respect to the
enforceability of provisions in the Indenture providing for specific
performance, injunctive relief or other equitable remedies, regardless of
whether such enforceability is sought in a proceeding in equity or at law. We
express no opinion concerning any law of any jurisdiction other than the laws of
the State of New York, the General Corporation Law of the State of Delaware and
the federal laws of the United States of America. Without limiting the
foregoing, we express no opinion with respect to matters of municipal laws or
the rules, regulations or orders of any municipal agencies within any state.

         Based upon and subject to the foregoing qualifications, assumptions and
limitations and the further limitations set forth below, it is our opinion that
the Exchange Notes have been duly authorized for issuance by the Company and,
when issued and delivered in exchange for the Existing Notes in the manner
described in the Registration Statement and when executed, issued and
authenticated in accordance with the terms of the Indenture, will constitute
valid and legally binding obligations of the Company, enforceable against the
Company in accordance with their terms.

         This opinion is limited to the specific issues addressed herein, and no
opinion may be inferred or implied beyond that expressly stated herein. We
assume no obligation to revise or supplement this opinion should the present
laws of the State of New York, the General Corporation Law of the State of
Delaware or the federal laws of the United States of America be changed by
legislative action, judicial decision or otherwise.

         We hereby consent to the filing of this letter as an exhibit to the
Registration Statement and to the reference to our firm under the heading "Legal
Matters" in the Registration Statement. In giving such consent, we do not admit
that we are in the category of persons whose consent is required under Section 7
of the Act or the rules and regulations of the Commission promulgated
thereunder.

         This opinion is furnished to you in connection with the filing of the
Registration Statement and is not to be used, circulated, quoted or otherwise
relied upon for any other purpose.

                                       Very truly yours,



                                       /s/ KELLEY DRYE & WARREN LLP