The opening words of the letter identify Paul and Timothy as the authors of this letter (1:1). But after the initial greeting the predominance of first person singular pronouns, the emphasis on Paul’s circumstances (1:12-26; 4:10-23), and the commendations of Timothy (2:19-24) and Epaphroditus (2:25-30) make it clear that Paul is the de facto author. Church fathers as early as Irenaeus (c. 130 – c. 200), Clement of Alexandria (c. 150–215), and Tertullian (160–240) attribute the letter to Paul. In his letter to this same church Polycarp (69–155) makes reference to Paul writing to the Philippians (Pol. Phil. 3:2), and at points seems to even echo the language of Paul’s letter to the Philippians. Pauline authorship has rarely been challenged even by the most critical of scholars.
Yet the role of Timothy should not be overlooked. He appears to have been converted under Paul’s ministry in Derbe and Lystra during Paul’s first missionary journey (Acts 14:8-23). On his return visit a couple of years later during his second missionary journey, Timothy joined Paul’s ministry team (Acts 16:1-5). Soon after Paul and Timothy—along with Luke and Silas—planted the church in Philippi (Acts 16:6-40). In the years to come Timothy became Paul’s most trusted ministry colleague, a kindred spirit who shared Paul’s heartfelt concern for the church (Phil 2:19-24). Paul sent him on various missions to maintain contact with and address problems in the various churches Paul had planted (Acts 17:14–15; 18:5; 19:22; 20:4; 1 Cor 16:10; 1 Thess 3:1-6). He was with Paul during his imprisonment in Rome (Phil 1:1; 2:19-24), no doubt serving as Paul’s link to various churches. At some point after Paul’s release from house arrest in Rome, Timothy and Paul were together in Ephesus for some time before Paul left for Macedonia (1 Tim 1:3). When Paul was eventually re-arrested and knew he was soon to be executed by Nero, he wrote his final (extant) letter to Timothy, asking him to visit him as soon as possible (2 Tim 4:9, 13, 21).
But if Paul is the de facto author of the letter, why does he mention Timothy in the salutation? While it is possible that Timothy acted as Paul’s scribe for this letter, there is no way of proving it. Based on what is known about how letters were written in the ancient world, it is certainly possible that Timothy provided input on the content of the letter.At the least Paul includes Timothy in the authorship of the letter to prepare the Philippians for the news that he will be sending Timothy to Philippi as soon as he sees how things go with him in Rome (Phil 2:19-24).
 Other Pauline letters list “co-authors” as well: 1 Corinthians (Sosthenes), 2 Corinthians (Timothy), Colossians (Timothy), 1-2 Thessalonians (Silvanus and Timothy), and Philemon (Timothy).
 According to a search in Bibleworks 9, there are 66 first person singular verb forms compared to just four first person plural forms. In each of those four verbs Paul is referring to all believers including himself (3:3, 15, 16, 20).
 Timothy is not mentioned in this account, but elsewhere Paul refers to him as his “true child in the faith” (1 Tim 1:2).
 For more on letter writing in the ancient world and the implications for studying the Pauline epistles, see E. Randolph Richards, Paul and First-Century Letter Writing: Secretaries, Composition, and Collection (Downers Grove: IVP, 2004)