Telepathy

Telepathy is a basic psionic power, possessed by some terrans, the zerg and the protoss.

Overview
Broadly speaking, telepathy involves the ability to read minds, sensing emotion/general thoughts, sending psychic messages  and accessing the memories of sentient creatures.

Protoss Telepathy

 * Main article: Protoss Mental Abilities

The protoss possess telepathy universally. Indeed, they are so proficient at it that during the Aeon of Strife, protoss could even prevent others from gaining access to their minds without technological devices. Protoss are able to read the minds of terrans with ease, but possess enough self control to not read such thoughtstreams.

The Khala-following protoss of present times cannot completely hide their thoughts from each other, but the ability can still be used to a degree. Protoss who do not follow the Khala (such as the Dark Templar) can completely shutter their minds to other Protoss.

Terran Telepathy
Terrans have the potentially to become a potent psionic race in only a few generations, although psychic abilities are currently rare. Terran psychic power is measured on the Psionic Index; most Terrans have a PI of 2 or less.

A few terran individuals possess telepathy. Most of these individuals become ghosts, who generally have a PI of 5 or 6. However, the degree of telepathy varies; the ability to sense surface thoughts is seemingly universal for such individuals although to directly read an individual's thoughts is not as common. Reading thoughts unintentionally is generally a given for telepaths and some are never able to completely filter external thoughts. This can create "feedback loops", where a person's thoughts is mirrored by the telepath. External fear becomes the telepath's fear, external anger becomes the telepath's anger, etc. However, with enough training, it is not impossible for a terran to prevent him/herself from doing so. In addition, non-telepaths, with enough willpower, can keep their thoughts to themselves.

To date, the only terran observed sending telepathic messages has been Devon Starke.

Zerg Telepathy
The zerg are entirely telepathic, but in a different manner to the protoss. While each protoss individual is his/her own self, the zerg utilize a mental heirarchy, with the Overmind(s) at the top, moving down to overlords, queens, and steadily down to lesser strains. These strains are capable of 'speech', but are incapable of ignoring the directives of higher strains. Although the Queen of Blades has taken the role of the Overminds at the top of the zerg heirarchy, the process remains the same.

To date, no thoughts of lesser strains have ever been known to be read by a protoss or terran telepath. The only known exception is Jim Raynor, who was able to hear the zerg 'speaking' whilst on Char. This was due to Kerrigan weilding her telepathy clumsily, and filtering her unwanted thoughts into Raynor whilst she came to accept who and what she had become. The protoss Zeratul was aware of this, but could not hear the zerg's thoughts.

Higher strains are a different story however, as Zeratul was able to communicate with the cerebrate Zasz via telepathy before assassinating the creature. Upon doing so, his mind connected with that of the Overmind, their thoughts being exchanged. The Overmind learnt of the location of the protoss homeworld of Aiur, while Zeratul learnt of the zerg's origins and goal of assimilating the protoss race.