Rules

Robot Combat Rules

Rules around building and competing with robots are focused on keeping things fair and safe. Robots can be dangerous so be sure to take precautions when building and testing your creations.

Robot Combat rules typically cover both robot construction and match procedures. There is no single governing body for Robot Combat which means each event can use whatever rules they like but the different rule sets have a lot of overlap. The first thing you should do before building your robot is to figure out which event you are building for and determine which rule set they use. Before you get too far into your robot make sure to know if the event you are building for have any exceptions for changes to the standard rules. Also be aware that any event organizer may disqualify a robot if they feel something about it is unsafe so if you think you've found a loophole or workaround to make your bot more destructive, clear it with the event organizer before you show up.

Sparc.tools has the most commonly used rules for Robot Construction and Match Procedures.

The Fighting Robot Association is centered in the UK and most events in Europe use their rules. The RFA rules are very similar to the Sparc.tools rules but there are a few differences so if you plan on competing in events across the two rule sets be sure that your robot will qualify.

The Norwalk Havoc Robot League has their own set of rules and have some differences in weight allowances for multi-bots.

Battlebots is a TV show that features 250 pound fighting robots and they have their own rules. Many of their rules are similar to the other rule sets but they have an active weapon requirement and further limitations on weapons for safety reasons.

Judging Systems

More detail and references are needed.

There are several different judging systems. Typically they take into consideration Aggression, Damage, and Control and assign points to each bot engaged in a match. SPARC uses such a judging system and have example videos with judges commentary that can help new judges understand how to assign scores.
Some events will weigh one category more than the others by assigning it more points. The intent here is to promote designs that excel in certain categories. Again the SPARC judging guidelines do this with giving Aggression (5 points), Damage (6 points) and Control (6 points) between two opponents.
In order to avoid ties, scored judging systems will have an odd number of total points resulting in a definite winner.
One down-side to the point system is that it can be possible for 2 of 3 judges to believe one robot has won but the 3rd judge assigns enough points to override them. Many events have done away with the detailed scoring system and simply have each judge indicate which bot they felt won the overall match. These systems can lead to more consistent results but some builders get frustrated without scores to indicate why they won or lost the match.
Different events also have different allowances for judges to talk with each other when making decisions. Some events want each judge to come to their own conclusions independently while others think it better to have judges point out things that they noticed and discuss the results.