Are Swift variables atomic?

It's very early to assume as no low-level documentation is available, but you can study from assembly. Hopper Disassembler is a great tool.

@interface ObjectiveCar : NSObject
@property (nonatomic, strong) id engine;
@property (atomic, strong) id driver;
@end

Uses objc_storeStrong and objc_setProperty_atomic for nonatomic and atomic respectively, where

class SwiftCar {
    var engine : AnyObject?    
    init() {
    }
}

uses swift_retain from libswift_stdlib_core and, apparently, does not have thread safety built in.

We can speculate that additional keywords (similar to @lazy) might be introduced later on.

Update 07/20/15: according to this blogpost on singletons swift environment can make certain cases thread safe for you, i.e.:

class Car {
    static let sharedCar: Car = Car() // will be called inside of dispatch_once
}

private let sharedCar: Car2 = Car2() // same here
class Car2 {

}

Update 05/25/16: Keep an eye out for swift evolution proposal https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md - it looks like it is going to be possible to have @atomic behavior implemented by yourself.

Swift has no language constructs around thread safety. It is assumed that you will be using the provided libraries to do your own thread safety management. There are a large number of options you have in implementing thread safety including pthread mutexes, NSLock, and dispatch_sync as a mutex mechanism. See Mike Ash's recent post on the subject: https://mikeash.com/pyblog/friday-qa-2015-02-06-locks-thread-safety-and-swift.html So the direct answer to your question of "Can I read and write to this variable in parallel safely?" is No.