How to transition to a new view controller with code only using Swift

This worked perfectly for me:

func switchScreen() {
    let mainStoryboard = UIStoryboard(name: "Main", bundle: Bundle.main)
    if let viewController = mainStoryboard.instantiateViewController(withIdentifier: "yourVcName") as? UIViewController {
        self.present(viewController, animated: true, completion: nil)
    }
}

SWIFT

Usually for normal transition we use,

let next:SecondViewController = SecondViewController()
self.presentViewController(next, animated: true, completion: nil)

But sometimes when using navigation controller, you might face a black screen. In that case, you need to use like,

let next:ThirdViewController = storyboard?.instantiateViewControllerWithIdentifier("ThirdViewController") as! ThirdViewController
self.navigationController?.pushViewController(next, animated: true)

Moreover none of the above solution preserves navigationbar when you call from storyboard or single xib to another xib. If you use nav bar and want to preserve it just like normal push, you have to use,

Let's say, "MyViewController" is identifier for MyViewController

let viewController = MyViewController(nibName: "MyViewController", bundle: nil)
self.navigationController?.pushViewController(viewController, animated: true)

Your code is just fine. The reason you're getting a black screen is because there's nothing on your second view controller.

Try something like:

secondViewController.view.backgroundColor = UIColor.redColor();

Now the view controller it shows should be red.

To actually do something with secondViewController, create a subclass of UIViewController and instead of

let secondViewController:UIViewController = UIViewController()

create an instance of your second view controller:

//If using code
let secondViewController = MyCustomViewController.alloc()

//If using storyboard, assuming you have a view controller with storyboard ID "MyCustomViewController"
let secondViewController = self.storyboard.instantiateViewControllerWithIdentifier("MyCustomViewController") as UIViewController

For those using a second view controller with a storyboard in a .xib file, you will want to use the name of the .xib file in your constructor (without the.xib suffix).

let settings_dialog:SettingsViewController = SettingsViewController(nibName: "SettingsViewController", bundle: nil)
self.presentViewController(settings_dialog, animated: true, completion: nil)

For anyone doing this on iOS8, this is what I had to do:

I have a swift class file titled SettingsView.swift and a .xib file named SettingsView.xib. I run this in MasterViewController.swift (or any view controller really to open a second view controller)

@IBAction func openSettings(sender: AnyObject) {
        var mySettings: SettingsView = SettingsView(nibName: "SettingsView", bundle: nil) /<--- Notice this "nibName" 
        var modalStyle: UIModalTransitionStyle = UIModalTransitionStyle.CoverVertical
        mySettings.modalTransitionStyle = modalStyle
        self.presentViewController(mySettings, animated: true, completion: nil)

    }

In Swift 2.0 you can use this method:

    let registrationView = LMRegistration()
    self.presentViewController(registrationView, animated: true, completion: nil)

Always use nibName file otherwise your preloaded content of Xib will not show .

vc : ViewController =  ViewController(nibName: "ViewController", bundle: nil) //change this to your class name

 self.presentViewController(vc, animated: true, completion: nil)

Updated for Swift 3, some of these answers are a bit outdated.

let mainStoryboard = UIStoryboard(name: "Main", bundle: Bundle.main)
        let vc : UIViewController = mainStoryboard.instantiateViewController(withIdentifier: "myStoryboardID") as UIViewController
        self.present(vc, animated: true, completion: nil)    }