Dermal application of various active substances is widely preferred for topical or systemic delivery. Solid lipid nanoparticles (SLNs) are colloidal drug carriers prepared by lipids which are in solid state at room temperature. SLNs attract considerable interest for topical applications, because they are prepared using non-irritant and non-toxic lipids and show excellent tolerability, good physical stability and high drug loading capacity. Semisolid SLNs are novel and promising approach for topical drug delivery. This book presents comprehensive information about the SLNs as a drug and cosmetic delivery system. The importance and the effectiveness of semisolid SLNs as a novel topical drug delivery system are also evaluated.