Adding the Observable type to TypeScript Interface

I would like to make a TypeScript interface like this:

declare namespace UserService {

    interface IUserService {
        // Error: "Observable" can't be found
        getUsers(): Observable<Array<UserService.IUser>>;
    }

    interface IUser {
        Id: number;
        FirstName: string;
        LastName: string;
    }
}

...and then use it like this

import { Injectable } from '@angular/core';
import { Http } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map'

@Injectable()
export class UserService implements UserService.IUserService {

    private usersUrl = 'http://localhost:12345/api/users';

    constructor(private http: Http) {}

    public getUsers(): Observable<Array<UserService.IUser>> {
        return this.http.get(this.usersUrl).map(response => response.json() as Array<UserService.IUser>);
    }
}

The problem is that the Observable type in the IUserService isn't found. If I use Promises and use Promise as the type then the Promise type is found but I want to use Observable.

It is possible I'm thinking about this wrong. I will accept a solution or an alternative.

Thank you