SS Officer: I just can't think of anything worse than a skull! Hans: A rat's- anus?! SS Officer: Yeah. And if we were fighting an army marching under… - Waffen-SS

In principle, the SS was to accept no new members after 1933, except from selected graduates of the Hitler Youth. However, the creation of the Waffen-SS and its rapid growth caused the partial suspension of this rule. However, service in the Waffen-SS did not necessarily include membership in the SS proper.
Prior to the war, suitable SS candidates were singled out while still in the Hitler Youth (HJ). Boys who had proved themselves, often under SS leadership, in the HJ patrol service were often tabbed for later SS service. If the candidate satisfied SS requirements in political reliability, racial purity, and physique, he was accepted as a candidate at the age of 18. At the annual Nazi Party Congress in September, candidates were accepted, received SS certificates, and were enrolled in the SS.
Service in the Waffen-SS was officially voluntary. The Waffen-SS claimed priority over all other branches of the armed forces in the selection of recruits. Eventually, to meet the high rate of casualties and the expansion of Waffen-SS field divisions, service in the Waffen-SS became compulsory for all members of the SS, and the voluntary transfer of personnel from any other branch of the armed forces was permitted. From 1943, pressure was exerted on members of the Hitler Youth to volunteer for the Waffen-SS. Later, entire Army, Navy, and Air Force units were taken over by the Waffen-SS, given SS training, and incorporated into field units. Waffen-SS enlistment drives in Germany were nearly continuous. Waffen-SS recruitment was regionally organized and controlled.

Believing that Germany would be victorious in war, SS chief Heinrich Himmler intended to strengthen the position of the SS relative to the established German elites after the war. To this end, Himmler persuaded Adolf Hitler in late 1939 to permit the establishment of an armed SS force known as the Waffen-SS.
Although initially restricted to four divisions, the Waffen-SS eventually fielded more than 20 divisions. This created an armed force of about half a million men and established a command and operations structure to rival the German army.
As military defeat reduced the prestige of the generals, the SS further encroached on the authority of the German armed forces (Wehrmacht). Already in 1942, the SS took over—from the armed forces—coordinating anti-partisan operations in the occupied Soviet Union. After the failure of the attempt on Hitler's life of July 20, 1944, Hitler appointed Himmler Commander of the Replacement Army (a position responsible for training and overseeing military personnel) and gave him command of matters relating to prisoners of war.

SS Officer: Hans, have you looked at our caps recently?
Hans: Our caps?
SS Officer: They've got skulls on them. [pause] Hans- are we the baddies?