Run-Length Limited (RLL) coding is used as modulation codes in virtually all optical disks (CD, DVD, Blu-Ray) and magnetic media, such as hard drives. It involves binary sequences that have a minimum and maximum length of same-polarity physical symbols. These two values (minimum and maximum run-length) are referred to as (d,k) RLL constraints. It’s theory dates back to Shannon himself, who first calculated the capacity of a channel under such constraints. DC-free, or – in general – minimum low-frequency content RLL codes is the most popular class of these codes. Immink designed the popular eight-to-forteen modulation (EFM) used by Compact Disks in the 80s and EFM+ used by DVDs in the 90s. The former is a block-code while the later is a state-dependent coding scheme. We will present the theory and background of RLL codes, together with some recent results regarding: state-dependent codebook optimization; enhanced error resilience through maximum likelihood and Viterbi decoding; an improved metric used for encoding; the effect of look-ahead encoding and the applicability of stack-algorithm in look-ahead encoding. Finally, a new theoretical result on (d,k) RLL coding channel capacity and the comparison to (d,k,c) RLL coding.