i-Orchestra is an iOS app that enables orchestral instrument simulation on a mobile device. It voices musical instrument samples by transmitting MIDI messages to the app via the on-screen MIDI keyboard or an external MIDI keyboard connected to the iOS device through the Apple Camera Connection Kit. The screen of the iOS device displays the musical score of the users' choice in pdf format, allowing the user to scroll over the score and play the score at the same time. A virtual orchestra can therefore be formed collaboratively by a group of users, each of whom individually simulates an orchestral part using the app.