SOURCE UNIT - Most cars come with CD players and even XM these days. Great! You can go for that ultra stealth look and use the stock radio. Oops. The stockers don't have preamp outputs. Nevermind. And line level converters aren't going to help your system sound any better. There are a handful of aftermarket head units with dual illumination, although not as many as there used to be. Most of the units are going with a multi-colored LCD anyway, so the dual color only applies to the buttons. MP3 compatibility is also pretty common, although if you really want to take it to the next level you can put a small PC or Mac Mini in the car and use it as a giant iPod. DRIVERS - Many manufacturers are putting separates in the front doors of their cars for better soundstage. This is helpful considering no self-respecting car audio guy is going to put coax speakers up front. Separates consist of a midrange driver, a tweeter, and a passive crossover (there are three way configurations that also use a midbass driver). You can drop a lot of cash on these, but they are the most important. A good idea is to buy the best separates that your budget will allow, then put somewhat comparable coaxials in the back (don't bother with triaxials). Another good idea is to use round drivers in the rear deck (with an adapter) instead of the ubiquitous 6x9 oval speakers. SUBWOOFER - There are many sub box configurations. They range from the most simple sealed designs to elaborate bandpass enclosures. Without getting into the physics of it all, it is really just a preference thing. Each type of box is typically conducive to a particular goal. For example, sealed boxes are tighter than ported ones, but ported boxes have better low frequency extension. Bandpass designs are great when space is tight. Speaker size is also relative, but anything over a 12" is really just pointless. Having a custom box built can be costly, so many companies offer high quality prefab units. A stealthy way to go is a spare tire enclosure as long as you have roadside assistance. AMPLIFIER - Many people think that speakers blow because of too much power. In reality, speakers blow due to distortion, which is caused by too little power for a desired volume level. The amp clips the tops and bottoms off of the waveform (i.e. "clipping"), the speakers distort, and boom. Head units with built-in amps are okay, but they tend to have high distortion levels and exaggerated power claims. While you could certainly go with an amp for your separates, an amp for the rear speakers, plus a sub amp, many manufacturers are making solid 5 channel amps that make for a simple but effective install. Depending on the amplifier it may be wiser to go with a 4 channel / sub amp combo. PROCESSOR - Not really a required piece of gear, processors can be convenient (plus they're a lot cooler than a graphic EQ). And as high resolution audio becomes popular, they will become more and more common as music is mixed in Dolby 5.1 Surround (although they'll likely built into the head unit). Processors are typically used to tune a system to fit a particular car's acoustics - which are usually terrible. Many are mounted out of the way with the amps while some are dash mounted. A great use for these units is having one in the trunk to tune the system flat, then having one in the dash with presets for things like windows up, windows down, maximum SPL, etc. ACTIVE CROSSOVER - Crossovers are often equated to traffic cops . This is pretty accurate, as they direct audio frequencies to the drivers that are best suited to produce those frequencies. Passive crossovers use no electronics and are typically found in aftermarket component groups. While these work great, they can sap power. Active xovers electronically control frequency routing. Some amps have onboard active xovers, but an external unit gives the most flexibility since the amp units typically use fixed crossover points. The crossover point is the transition from one driver to another, such as setting your mids to crossover at 2.5 kHz. The overlap (or rolloff) can also be adjusted. CAPACITOR - Transients are sudden soft to loud changes in music (basically speaking). When one of these transients is in the lower frequencies, your amp has the tendency to choke because it doesn't have the ability to anticipate such a sudden need for extra power. Low frequencies are the most demanding on an amplifier and the most difficult to produce at high db levels, and a capacitor (also called a stiffening capacitor) stores a bit of current in reserve that the amplifier can call upon when a bass transient hits. They are available in different ratings (measured in Farads) depending on the size of your subwoofers and their power handling capabilities. Check the specs! RADAR DETECTOR - Everybody says they're going to buy a radar detector, and then they never do. Until they get a ticket. Traffic enforcement is a cat and mouse game, with revenue being the motivating factor more than safety. Without a detector, you're a sitting duck. Most detectors offer multiple modes that include X (door openers), K (could be a smokey), and Ka (definitely a smokey). Laser is usually an option as well, but less useful since the cop can pinpoint your car in traffic before you even see him. Units such as the Valentine One, though expensive, offer front and rear facing detectors. This enables you to know where the threat is coming from and react accordingly. SCANNER - If the radar detector isn't enough protection, there is always a mobile scanner. You might want to tuck it away somewhere though. It isn't that they are illegal, but they are likely to upset an officer if you do get pulled over. These units offer several features, including Highway Patrol Scan, Local Police Scan, and Fire / Emergency Scan. Uniden also offers a Highway Patrol Alert, which lets you know when you are within three miles of a State Patrol unit using a mobile extender. Many units will also alert you to Park Rangers in the vicinity, which can be very handy when carving mountain roads that happen to be in a national park. Don't be a victim of a police fund raiser!