There are two downloads available for Windows users.
Windows 64-bit with JDK 8 Included: This archive includes both SQL Developer and an embedded copy of the Java 8 Development Kit (JDK). Simply extract the zip to a fresh directory and run the sqldeveloper.exe in the top directory. The EXE is configured to run the embedded JDK by default.
Note: the Windows EXE requires a MSVCR100.dll to run. Most computers will already have this file and in the Windows PATH. However, if the first copy of the file found by the EXE is a 32-bit copy of the DLL, then SQL Developer will fail to start. You can fix this by copying a 64-bit version of the DLL into the BIN directory or updating your OS PATH such that a 64 bit copy of the DLL is found first.
Windows 32-bit/64-bit: This archive. will work on a 32 or 64 bit Windows OS. The bit level of the JDK you install will determine if it runs as a 32 or 64 bit application. This download does not include the required Java 8 JDK. You will need to install it if it's not already on your machine.
If SQL Developer cannot find Java on your machine, it will prompt you for the path for a JDK home. This only occurs the first time you launch SQL Developer. A valid Java Home on Windows will be similar to C:\Program Files\Java\jdk1.8.0_45