Can you upgrade the processor in the Retina Display MacBook Pro models?

No. The processor in all MacBook Pro models -- Retina Display or otherwise -- is soldered in place and cannot be upgraded after purchase.

However, at the time of purchase, some Retina Display MacBook Pro models can be equipped with faster processors. Specifically, these Retina Display MacBook Pro models were offered at the time of purchase with a faster processor option for the price listed:

13-Inch Retina Display MacBook Pro Custom Processors

MacBook Pro	Subfamily	Processor Option	Price
"Core i5" 2.5 13"	Late 2012	2.9 GHz Core i7 (I7-3520M)	US$200
"Core i5" 2.6 13"	Early 2013	3.0 GHz Core i7 (I7-3540M)	US$200

15-Inch Retina Display MacBook Pro Custom Processors

MacBook Pro	Subfamily	Processor Option	Price
"Core i7" 2.3 15"	Mid-2012	2.7 GHz Core i7 (I7-3820QM)	US$350
"Core i7" 2.6 15"	Mid-2012	2.7 GHz Core i7 (I7-3820QM)	US$250
"Core i7" 2.4 15"*	Early 2013	2.8 GHz Core i7 (I7-3840QM)	US$350
"Core i7" 2.7 15"	Early 2013	2.8 GHz Core i7 (I7-3840QM)	US$350

*This model also can be upgraded at the time of purchase with a 2.7 GHz Core i7 (I7-3740QM) for an additional US$100.

For those not familiar with Mac history, the PowerBook G3 line is the last Apple notebook series to have upgradable processors. The PowerBook G3 models have processors mounted on a removable card and EveryMac.com still provides a list of upgrade cards compatible with these now vintage Macs.

How do you upgrade the RAM in the Retina Display MacBook Pro models? What type of RAM do they use? How much RAM do they actually support? Is it even possible to upgrade the RAM?

By default, both the 13-Inch and 15-Inch Retina Display MacBook Pro models have 8 GB of 1600 MHz DDR3L SDRAM soldered onboard. The 15-Inch models can be upgraded at the time of purchase to 16 GB of RAM as a US$200 option, but they cannot cannot be upgraded at all after purchase. Even worse, the 13-Inch Retina Display MacBook Pro models cannot be upgraded period, either at the time of purchase or otherwise.

Although 8 GB of RAM likely is sufficient for most users at the moment, EveryMac.com would strongly recommend upgrading the 15-Inch Retina Display MacBook Pro model to 16 GB of RAM at the time of purchase to prolong the usable life of the notebook. Along with in-depth performance testing, the MacPerformaceGuide sharply, but wisely, took Apple to task for originally concealing that RAM cannot be upgraded after purchase. Apple has more recently remedied this omission in the company's store.

If having the ability to upgrade the RAM yourself after purchase is important to you, you might instead prefer to buy a traditional 13-Inch or 15-Inch "Mid-2012" MacBook Pro model -- which actually are capable of supporting 16 GB of RAM regardless of display size -- or alternately, you might feel compelled to purchase a notebook from a company other than Apple.

Is it possible to add a second SSD or hard drive to a Retina Display MacBook Pro?

No. Many earlier MacBook Pro models can support a second SSD or hard drive in the optical drive bay by using a product like site sponsor OWC's Data Doubler.

However, as none of the Retina Display MacBook Pro models have an optical drive, it is not possible to add a second internal SSD or a hard drive.

What kind of expansion slots are provided by the Retina Display MacBook Pro models?

The Retina Display-equipped MacBook Pro models only have a single SDXC card slot. They do not have an ExpressCard/34 expansion slot.

If an ExpressCard/34 expansion slot is important to you, you might instead prefer to purchase an earlier 15" or 17" MacBook Pro model that is appropriately equipped. All 17" MacBook Pro models have an ExpressCard/34 expansion slot, but many 15" models do not. The fastest MacBook Pro models with ExpressCard/34 slots are the MacBook Pro "Core i7" 2.4 17" (Late 2011) and its custom order variant, the MacBook Pro "Core i7" 2.5 17" (Late 2011).

However, many other models with ExpressCard/34 capability also are available. EveryMac.com's Ultimate Mac Sort feature makes it simple to view a complete list.